Merge feat/document-language: Petal follows the writing into her own language

Phase 28 whole — corrections and explanations in the document's
language, the translate card pointed the right way, and a garden that
knows which language each card is in.

Claude-Session: https://claude.ai/code/session_01GJHNvirh7Hzhc9RL3HAvz7

internal/db/db.go

@@ -589,6 +589,55 @@ CREATE INDEX idx_suggestions_resolved ON suggestions(status, resolved_at);
 			stmt: `
 ALTER TABLE users ADD COLUMN direction TEXT NOT NULL DEFAULT 'learning_en'
     CHECK(direction IN ('learning_en','learning_pair'));
+`,
+		},
+		{
+			// Which language this document is written in — 'en' or 'pair'.
+			//
+			// It is stored, rather than recomputed per pass and forgotten, for one
+			// reason: the verdict has hysteresis (see suggestions/doclang.go). A
+			// bilingual document sits between the two thresholds, and "whatever it
+			// was last time" is only an answer if last time was written down. Without
+			// the column a mixed paragraph would alternate its cards' language
+			// between passes.
+			//
+			// 'pair' rather than a language code, deliberately. Which language "pair"
+			// names is the owner's users.pair_lang, so changing her pair re-reads her
+			// documents instead of stranding a stale language name on every one of
+			// them.
+			//
+			// Empty is the backfill and means English: every document that exists
+			// today was written by a Mandarin native practising English, and English
+			// is what every surface assumed before this phase.
+			name: "0017_document_lang",
+			stmt: `
+ALTER TABLE documents ADD COLUMN doc_lang TEXT NOT NULL DEFAULT ''
+    CHECK(doc_lang IN ('', 'en', 'pair'));
+`,
+		},
+		{
+			// Which language a garden card is in — the same '' | 'en' | 'pair'
+			// vocabulary as documents.doc_lang, and set from it: a word is captured
+			// (or a phrase planted) out of a document, so the document's verdict is
+			// the card's language. A card with no document keeps '', which reads as
+			// English like every other empty here.
+			//
+			// The garden needed this the moment a document could be written in her
+			// own language. Before Phase 28 every card was English by construction;
+			// now a Portuguese lookup lands beside an English one with nothing to
+			// tell them apart, and two surfaces get it wrong without the tag — the
+			// review card's read-aloud (which would say a Portuguese word in a US
+			// English voice) and the panel, where a mixed garden is illegible.
+			//
+			// Every card is reviewed regardless. Filtering the queue to the half she
+			// is learning was the alternative and is wrong for the writer this is
+			// for: the words she met while writing Portuguese are still words she
+			// met, and a garden that quietly drops them is a garden that stops being
+			// a record of her reading.
+			name: "0018_vocab_lang",
+			stmt: `
+ALTER TABLE vocab_words ADD COLUMN lang TEXT NOT NULL DEFAULT ''
+    CHECK(lang IN ('', 'en', 'pair'));
 `,
 		},
 	}

internal/db/models.go

@@ -41,6 +41,11 @@ type Document struct {
 	ContentText string `json:"content_text"` // plain text for the LLM
 	Tone        string `json:"tone"`         // target writing tone; steers LLM advice
 	WordCount   int    `json:"word_count"`
+	// DocLang is which language this document is written in — '' | 'en' | 'pair'
+	// (migration 0017), written by the checkpoint pass and never by the client.
+	// It reaches the client read-only, for the one decision the client has to
+	// make on its own: which voice reads a selection aloud. '' means English.
+	DocLang   string    `json:"doc_lang"`
 	CreatedAt time.Time `json:"created_at"`
 	UpdatedAt time.Time `json:"updated_at"`
 

internal/docs/handlers.go

@@ -225,13 +225,14 @@ func (h *Handler) fetch(userID, id string) (db.Document, error) {
 	var doc db.Document
 	err := h.DB.QueryRow(
 		`SELECT id, user_id, title, content, content_text, tone, word_count,
-		        created_at, updated_at, preserve_history
+		        created_at, updated_at, preserve_history, doc_lang
 		   FROM documents
 		  WHERE id = ? AND user_id = ?`,
 		id, userID,
 	).Scan(
 		&doc.ID, &doc.UserID, &doc.Title, &doc.Content, &doc.ContentText,
 		&doc.Tone, &doc.WordCount, &doc.CreatedAt, &doc.UpdatedAt, &doc.PreserveHistory,
+		&doc.DocLang,
 	)
 	return doc, err
 }

internal/llm/checkpoint.go

@@ -41,9 +41,9 @@ type checkpointResponse struct {
 // RunCheckpoint sends the grammar checkpoint and parses the JSON result. It
 // applies the latency-guard truncation and the checkpoint sampling parameters
 // from the spec.
-func RunCheckpoint(ctx context.Context, client LLMClient, contentText, tone string, _ Lang) ([]RawSuggestion, error) {
+func RunCheckpoint(ctx context.Context, client LLMClient, contentText, tone string, t Target) ([]RawSuggestion, error) {
 	raw, err := client.Complete(ctx, CompletionRequest{
-		Messages:          CheckpointMessages(TruncateDoc(contentText), tone),
+		Messages:          CheckpointMessages(TruncateDoc(contentText), tone, t),
 		MaxTokens:         checkpointMaxTokens,
 		Temperature:       0.3,
 		RepetitionPenalty: 1.15,

internal/llm/collocation.go

@@ -20,9 +20,9 @@ const CollocationInterval = 25 * time.Second
 // The tone argument is accepted for a uniform pass signature and passed through
 // to the prompt so a hint can prefer a register-appropriate pairing. `lang` is
 // the writer's pair language — the one each hint's short gloss is written in.
-func RunCollocation(ctx context.Context, client LLMClient, contentText, tone string, lang Lang) ([]RawSuggestion, error) {
+func RunCollocation(ctx context.Context, client LLMClient, contentText, tone string, t Target) ([]RawSuggestion, error) {
 	raw, err := client.Complete(ctx, CompletionRequest{
-		Messages:          CollocationMessages(contentText, tone, lang),
+		Messages:          CollocationMessages(contentText, tone, t),
 		MaxTokens:         2048,
 		Temperature:       0.3,
 		RepetitionPenalty: 1.15,

internal/llm/lang_test.go

@@ -31,7 +31,7 @@ func TestLangForFallsBackToDefault(t *testing.T) {
 func TestPromptsNameTheWritersLanguage(t *testing.T) {
 	pt := LangFor("pt-PT")
 
-	collocation := CollocationMessages("The rain was strong.", "casual", pt)[0].Content
+	collocation := CollocationMessages("The rain was strong.", "casual", EnglishTarget(pt))[0].Content
 	if !strings.Contains(collocation, "European Portuguese") {
 		t.Fatalf("collocation prompt doesn't ask for a pt-PT gloss:\n%s", collocation)
 	}
@@ -73,7 +73,7 @@ func TestPromptsNameTheWritersLanguage(t *testing.T) {
 func TestDefaultPairStillReadsAsBefore(t *testing.T) {
 	zh := LangFor("zh")
 
-	if got := CollocationMessages("x", "", zh)[0].Content; !strings.Contains(got, "Simplified Chinese (Mandarin) gloss in parentheses") {
+	if got := CollocationMessages("x", "", EnglishTarget(zh))[0].Content; !strings.Contains(got, "Simplified Chinese (Mandarin) gloss in parentheses") {
 		t.Fatalf("zh collocation gloss changed:\n%s", got)
 	}
 	if got := TranslateMessages("x", zh)[0].Content; !strings.Contains(got, "natural, friendly Simplified Chinese (Mandarin)") {

internal/llm/prompts.go

@@ -45,11 +45,55 @@ func toneGuidance(tone string) string {
 		"be improved, prefer suggestions that fit that tone, and gently flag wording that clashes with it."
 }
 
+// pairCheckpointSystemPrompt is the grammar checkpoint for a document written in
+// the writer's own language rather than in English.
+//
+// It is a separate constant rather than a language clause appended to
+// checkpointSystemPrompt, because that prompt opens by naming the reader as an
+// ESL learner and asks for "common ESL patterns" — appending "and explain in
+// Portuguese" would hand the model two contradictory framings. Only the framing
+// differs; the JSON contract and the tone clause below it are the same
+// instructions in the same order, so the two prompts stay comparable.
+//
+// The "never translate" line is the one the model most wants to disobey: asked
+// to improve Portuguese while being an English writing assistant by training, it
+// will happily hand back an English rendering, which is a translation card
+// (Phase 25's `isTranslation`) and not a correction.
+const pairCheckpointSystemPrompt = `You are a warm, encouraging writing assistant. The person you are helping is ` +
+	`writing in %[1]s, and the text below is %[1]s. ` +
+	`Analyze it and identify up to 5 issues: grammar errors, unnatural phrasing, ` +
+	`incorrect idiom usage, or unclear sentences.
+
+Both "original" and "replacement" must be written in %[1]s. You are improving their %[1]s writing — ` +
+	`never translate it into English, and never suggest they write in English instead.
+Write every "explanation" in %[2]s.
+
+Be specific, friendly, and explain WHY each suggestion improves the writing.%[3]s
+
+Respond ONLY with valid JSON. No preamble, no markdown fences. Format:
+{
+  "suggestions": [
+    {
+      "original": "exact text from the document that needs fixing",
+      "replacement": "corrected version",
+      "explanation": "friendly one-sentence explanation",
+      "type": "grammar|phrasing|idiom|clarity"
+    }
+  ]
+}
+
+If the writing looks good, return: {"suggestions": []}`
+
 // CheckpointMessages builds the message array for a grammar checkpoint over the
-// given (already-truncated) document text, steered toward the document's tone.
-func CheckpointMessages(contentText, tone string) []Message {
+// given (already-truncated) document text, steered toward the document's tone
+// and aimed at the language the document is actually written in.
+func CheckpointMessages(contentText, tone string, t Target) []Message {
+	system := fmt.Sprintf(checkpointSystemPrompt, toneGuidance(tone))
+	if t.Flipped() {
+		system = fmt.Sprintf(pairCheckpointSystemPrompt, t.Correct.Name, t.Explain.Name, toneGuidance(tone))
+	}
 	return []Message{
-		{Role: "system", Content: fmt.Sprintf(checkpointSystemPrompt, toneGuidance(tone))},
+		{Role: "system", Content: system},
 		{Role: "user", Content: contentText},
 	}
 }
@@ -82,12 +126,48 @@ Respond ONLY with valid JSON. No preamble, no markdown fences. Format:
 
 If the voice is consistent throughout, return: {"suggestions": []}`
 
+// pairVoiceSystemPrompt is the voice pass for a document in the writer's own
+// language. Voice consistency is the one pass that transfers across languages
+// unchanged — a paragraph that reads as pasted from elsewhere reads that way in
+// any language — so only the framing and the explanation language move.
+const pairVoiceSystemPrompt = `You are a warm, encouraging writing assistant. The person you are helping is writing ` +
+	`in %[1]s. You are reviewing a COMPLETE %[1]s document for VOICE CONSISTENCY only — not grammar.
+
+Read the whole document to learn the writer's natural voice, then identify any passages (2 or more sentences) ` +
+	`that feel tonally inconsistent with the surrounding writing — unusually formal, unusually polished, or phrased ` +
+	`in a way that differs from the writer's established voice elsewhere in the document. These often signal text ` +
+	`that was paraphrased too closely from another source. Do not flag the first paragraph (there is no baseline yet). ` +
+	`Do not flag grammar or spelling mistakes — only voice.
+
+Quote each passage exactly as it appears, in %[1]s. Write every "explanation" in %[2]s.
+
+Respond ONLY with valid JSON. No preamble, no markdown fences. Format:
+{
+  "suggestions": [
+    {
+      "original": "exact passage from the document that feels inconsistent",
+      "replacement": null,
+      "explanation": "friendly one-sentence note about why this passage sounds unlike the rest",
+      "type": "voice"
+    }
+  ]
+}
+
+If the voice is consistent throughout, return: {"suggestions": []}`
+
 // VoiceMessages builds the message array for a voice-consistency pass. Unlike
 // the checkpoint, the caller passes the WHOLE document (no truncation) — voice
 // consistency is judged against the established voice everywhere else.
-func VoiceMessages(contentText string) []Message {
+//
+// The pass had no language argument at all before Phase 28, which was the same
+// English assumption the checkpoint made, just unstated.
+func VoiceMessages(contentText string, t Target) []Message {
+	system := voiceSystemPrompt
+	if t.Flipped() {
+		system = fmt.Sprintf(pairVoiceSystemPrompt, t.Correct.Name, t.Explain.Name)
+	}
 	return []Message{
-		{Role: "system", Content: voiceSystemPrompt},
+		{Role: "system", Content: system},
 		{Role: "user", Content: contentText},
 	}
 }
@@ -133,10 +213,17 @@ If every pairing already sounds natural, return: {"suggestions": []}`
 // CollocationMessages builds the message array for a collocation pass over the
 // WHOLE document (no truncation), gently steered toward the document's tone so a
 // hint can prefer a register-appropriate pairing. The parenthetical gloss is
-// written in the writer's own language.
-func CollocationMessages(contentText, tone string, lang Lang) []Message {
+// written in the writer's own language — `Pair`, not `Explain`: the gloss is
+// addressed to her rather than to the document.
+//
+// The coach itself remains English-only. Collocation lists are the one thing
+// here that is genuinely per-language knowledge rather than framing, and
+// "natives usually say" for Portuguese is a claim this prompt has no grounds to
+// make yet; a flipped document simply gets the pass it always got. (Phase 28
+// moved the checkpoint and the voice pass; this one waits for evidence.)
+func CollocationMessages(contentText, tone string, t Target) []Message {
 	return []Message{
-		{Role: "system", Content: fmt.Sprintf(collocationSystemPrompt, toneGuidance(tone), lang.Name)},
+		{Role: "system", Content: fmt.Sprintf(collocationSystemPrompt, toneGuidance(tone), t.Pair.Name)},
 		{Role: "user", Content: contentText},
 	}
 }

internal/llm/target.go

@@ -0,0 +1,54 @@
+package llm
+
+// Which language a pass corrects, and which language it explains in.
+//
+// Until Phase 28 there was no question to answer: every prompt was written
+// around English prose explained in English, and the pair language reached her
+// only when she asked for it (Ask Petal, the explanation translator). That is
+// the right default for a writer practising English and the wrong one for a
+// document she wrote in her own language, where Petal would read Portuguese,
+// say nothing about it, and file a mechanics note about the one English
+// sentence at the end.
+//
+// The two fields are two different decisions reading two different pieces of
+// state, and collapsing them would be the bug:
+//
+//   - Correct follows the DOCUMENT. Portuguese prose gets Portuguese
+//     corrections; that is the whole point.
+//   - Explain follows the WRITER — the half of her pair she is *not* learning
+//     (users.direction), because an explanation is teaching, and teaching lands
+//     in the language she reads most easily.
+//
+// Today those two coincide for every account that exists: `learnerPairs` is
+// {"zh"}, so fr, es and pt-PT writers are all `learning_en` and their
+// non-learned half *is* the pair language. That equality is a fact about
+// today's roster, not about the design — the same shape of assumption that had
+// to be unpicked from `pair_lang` in migration 0016. Keep them apart.
+type Target struct {
+	// Correct is the language the writing is in, and so the language both
+	// `original` and `replacement` must be written in.
+	Correct Lang
+	// Explain is the language each explanation is written in.
+	Explain Lang
+	// Pair is the writer's pair language regardless of what this document is
+	// written in. The collocation coach's parenthetical gloss is addressed to
+	// her rather than to the document, so it reads this and not Correct.
+	Pair Lang
+}
+
+// English as the prompts name it. Not in `langs`: that map answers "which
+// language is the writer's half of the pair", and English is the constant on
+// the other side of every pair Petal supports.
+var English = Lang{Code: "en", Name: "English", Why: "why"}
+
+// EnglishTarget is the pre-Phase-28 behaviour, made explicit: an English
+// document, corrected and explained in English, for a writer whose pair
+// language is `pair`. Every existing user is on this path and the prompt it
+// produces is byte-identical to the one that shipped before this phase.
+func EnglishTarget(pair Lang) Target {
+	return Target{Correct: English, Explain: English, Pair: pair}
+}
+
+// Flipped reports whether this document is in the pair language rather than in
+// English — i.e. whether the pass is reading her own language.
+func (t Target) Flipped() bool { return t.Correct.Code != English.Code }

internal/llm/target_test.go

@@ -0,0 +1,115 @@
+package llm
+
+import (
+	"strings"
+	"testing"
+)
+
+// The prompt every account is on today, written out in full.
+//
+// Phase 28 gave the checkpoint a second framing for documents in the writer's
+// own language, and the risk of that change is not that the new prompt is wrong
+// — it is that the old one moved by a word while nobody was looking. Every user
+// who exists is a Mandarin native writing English, so this string is the one
+// Petal actually sends, all day. It is duplicated here on purpose: a golden
+// copied from the constant it guards guards nothing.
+const goldenEnglishCheckpointPrompt = `You are a warm, encouraging writing assistant helping someone who speaks English as a second language. Analyze the text below and identify up to 5 issues: grammar errors, unnatural phrasing, incorrect idiom usage, or unclear sentences that are common ESL patterns.
+
+Be specific, friendly, and explain WHY each suggestion improves the writing.
+
+Respond ONLY with valid JSON. No preamble, no markdown fences. Format:
+{
+  "suggestions": [
+    {
+      "original": "exact text from the document that needs fixing",
+      "replacement": "corrected version",
+      "explanation": "friendly one-sentence explanation",
+      "type": "grammar|phrasing|idiom|clarity"
+    }
+  ]
+}
+
+If the writing looks good, return: {"suggestions": []}`
+
+func TestEnglishDocumentPromptIsUnchanged(t *testing.T) {
+	msgs := CheckpointMessages("I has two apple.", "", EnglishTarget(LangFor("zh")))
+	if got := msgs[0].Content; got != goldenEnglishCheckpointPrompt {
+		t.Fatalf("the English checkpoint prompt moved:\n--- got ---\n%s\n--- want ---\n%s", got, goldenEnglishCheckpointPrompt)
+	}
+	if msgs[1].Content != "I has two apple." {
+		t.Fatalf("document text mangled: %q", msgs[1].Content)
+	}
+
+	// The tone clause still lands, in the same place it always did.
+	toned := CheckpointMessages("x", "academic", EnglishTarget(LangFor("zh")))[0].Content
+	if !strings.Contains(toned, "formal, academic, and objective") {
+		t.Fatalf("English checkpoint lost its tone guidance:\n%s", toned)
+	}
+}
+
+// A document in her own language gets a prompt that names that language, keeps
+// the corrections inside it, and drops the framing that only makes sense when
+// the thing being written is English.
+func TestFlippedCheckpointPrompt(t *testing.T) {
+	pt := LangFor("pt-PT")
+	system := CheckpointMessages("Hoje foi um dia bom.", "casual", Target{Correct: pt, Explain: pt, Pair: pt})[0].Content
+
+	if !strings.Contains(system, "European Portuguese") {
+		t.Fatalf("flipped checkpoint doesn't name the language:\n%s", system)
+	}
+	if strings.Contains(system, "second language") || strings.Contains(system, "ESL") {
+		t.Fatalf("flipped checkpoint kept the ESL framing:\n%s", system)
+	}
+	if !strings.Contains(system, "never translate it into English") {
+		t.Fatalf("flipped checkpoint doesn't forbid translating:\n%s", system)
+	}
+	// The shared contract below the framing has to survive the split.
+	for _, want := range []string{`"suggestions"`, `"replacement"`, "grammar|phrasing|idiom|clarity", "relaxed, friendly, and conversational"} {
+		if !strings.Contains(system, want) {
+			t.Fatalf("flipped checkpoint dropped %q:\n%s", want, system)
+		}
+	}
+	if strings.Contains(system, "%!") {
+		t.Fatalf("flipped checkpoint has a formatting error:\n%s", system)
+	}
+}
+
+// The two decisions are separate arguments and must reach the prompt separately:
+// corrections in the document's language, the explanation in the language she
+// reads most easily. Only the zh pair can be travelled both ways today, so it is
+// the only one that can prove they haven't been quietly collapsed into one.
+func TestFlippedPromptsExplainInTheirOwnLanguage(t *testing.T) {
+	zh := LangFor("zh")
+
+	// Native Mandarin, practising English, writing Chinese: both halves Chinese.
+	both := CheckpointMessages("今天天气很好。", "", Target{Correct: zh, Explain: zh, Pair: zh})[0].Content
+	if strings.Count(both, "Simplified Chinese (Mandarin)") < 2 {
+		t.Fatalf("expected corrections and explanations both in Chinese:\n%s", both)
+	}
+	if strings.Contains(both, "explanation"+`" in English`) {
+		t.Fatalf("explanation language leaked to English:\n%s", both)
+	}
+
+	// Native English, learning Chinese, writing Chinese: Chinese corrections,
+	// English explanations.
+	split := CheckpointMessages("今天天气很好。", "", Target{Correct: zh, Explain: English, Pair: zh})[0].Content
+	if !strings.Contains(split, `Write every "explanation" in English.`) {
+		t.Fatalf("learner direction didn't get English explanations:\n%s", split)
+	}
+	if !strings.Contains(split, "writing in Simplified Chinese (Mandarin)") {
+		t.Fatalf("learner direction lost its Chinese corrections:\n%s", split)
+	}
+
+	// Same for the voice pass, which had no language at all before this phase.
+	voice := VoiceMessages("今天天气很好。", Target{Correct: zh, Explain: English, Pair: zh})[0].Content
+	if !strings.Contains(voice, `Write every "explanation" in English.`) || !strings.Contains(voice, "Simplified Chinese") {
+		t.Fatalf("flipped voice prompt got its languages wrong:\n%s", voice)
+	}
+	if strings.Contains(voice, "second language") {
+		t.Fatalf("flipped voice prompt kept the ESL framing:\n%s", voice)
+	}
+	// An English document still gets exactly the voice prompt it always got.
+	if got := VoiceMessages("x", EnglishTarget(zh))[0].Content; got != voiceSystemPrompt {
+		t.Fatalf("English voice prompt moved:\n%s", got)
+	}
+}

internal/llm/voice.go

@@ -19,9 +19,9 @@ const VoiceInterval = 20 * time.Second
 // The tone argument is accepted for a uniform pass signature but ignored: voice
 // consistency is judged against the document's own established voice, not an
 // externally-chosen register.
-func RunVoice(ctx context.Context, client LLMClient, contentText, _ string, _ Lang) ([]RawSuggestion, error) {
+func RunVoice(ctx context.Context, client LLMClient, contentText, _ string, t Target) ([]RawSuggestion, error) {
 	raw, err := client.Complete(ctx, CompletionRequest{
-		Messages:          VoiceMessages(contentText),
+		Messages:          VoiceMessages(contentText, t),
 		MaxTokens:         2048,
 		Temperature:       0.3,
 		RepetitionPenalty: 1.15,

internal/suggestions/doclang.go

@@ -0,0 +1,179 @@
+package suggestions
+
+import "strings"
+
+// What language is this DOCUMENT in — as opposed to this span.
+//
+// `readsAsPairLang` next door answers a span-level question for the purpose of
+// labelling one card, and it is written to under-claim: two marker words, or
+// nothing. A whole document needs the opposite temperament. A proportion, not a
+// presence — one Portuguese quotation inside an English essay must not flip the
+// entire pass into Portuguese, and one English sentence at the end of a
+// Portuguese journal must not keep it in English.
+//
+// Three properties, in the order they bite:
+//
+//   - Decided over the WHOLE document, never a chunk. The grammar checkpoint
+//     sends only the sentences that changed, so a verdict computed from what it
+//     asked about would put an English card in a Portuguese journal the moment
+//     she edits its one English line. The caller passes content_text, always.
+//
+//   - Hysteresis. A bilingual paragraph sits near whatever single threshold we
+//     pick, and a document crossing it every few keystrokes would alternate card
+//     languages between passes — the same instability the mascot needed a band
+//     for. Flip to the pair at 70% and back only below 40%; in between, whatever
+//     it was last time stands. The band is the feature, not a rounding
+//     tolerance.
+//
+//   - Plain code, no model call. The house rule is that the LLM is garnish,
+//     never a gatekeeper: a document must not become uncheckable because the
+//     inference box is down.
+const (
+	docLangEnglish = "en"
+	docLangPair    = "pair"
+)
+
+// The band. Deliberately wide: the cost of an unnecessary flip is every card in
+// the document changing language, which is far more startling than a paragraph
+// of mixed writing being read as whichever language it was a minute ago.
+const (
+	flipToPairAt       = 0.70
+	flipToEnglishBelow = 0.40
+)
+
+// documentLang returns the language verdict for a document, given the verdict it
+// carried before. `prev` is "" for a document that has never been read.
+//
+// The result is one of docLangEnglish / docLangPair — not a language code. Which
+// language "pair" means is the writer's `pair_lang`, and keeping the stored
+// verdict relative to her pair means changing her pair doesn't strand a stale
+// language name on every document she owns.
+func documentLang(contentText, pairLang, prev string) string {
+	if prev != docLangPair {
+		prev = docLangEnglish
+	}
+	p := normalizePairLang(pairLang)
+	if !hasLangTest(p) {
+		// No test for this pair: say English, which is what every surface did
+		// before this phase. A wrong flip is louder than a missing one.
+		return docLangEnglish
+	}
+
+	var pair, english int
+	for _, c := range splitChunks(contentText, "") {
+		switch sentenceLang(c.text, p) {
+		case docLangPair:
+			pair++
+		case docLangEnglish:
+			english++
+		}
+	}
+	decided := pair + english
+	if decided == 0 {
+		// Nothing to go on — an empty document, a list of numbers, a title. Hold
+		// the previous verdict rather than resetting a Portuguese journal to
+		// English because she cleared it to start again.
+		return prev
+	}
+
+	ratio := float64(pair) / float64(decided)
+	switch {
+	case ratio >= flipToPairAt && corroborated(contentText, p):
+		return docLangPair
+	case ratio < flipToEnglishBelow:
+		return docLangEnglish
+	default:
+		return prev
+	}
+}
+
+// sentenceLang classifies one sentence as pair-language, English, or neither.
+//
+// Neither is a real answer and carries weight: a sentence with no evidence
+// either way ("Bom dia.", "OK.", a heading) is left out of the ratio entirely
+// rather than counted for the language it isn't. Counting the undecided as
+// English is what would keep a Portuguese document in English forever, since
+// short sentences carry no markers.
+func sentenceLang(s, p string) string {
+	if p == "zh" {
+		han, latin := scriptCounts(s)
+		switch {
+		case han >= 2 && han > latin:
+			return docLangPair
+		case latin >= 3 && latin > han:
+			return docLangEnglish
+		}
+		return ""
+	}
+	// A Latin pair shares its alphabet with English, so both sides are counted
+	// the same way and the larger pile of evidence wins. A tie — including no
+	// evidence at all — is no answer, which is why the English list below is
+	// curated as carefully against the pair languages as theirs is against
+	// English.
+	pair := distinctMarkers(s, latinMarkers[p])
+	eng := distinctMarkers(s, englishMarkers)
+	switch {
+	case pair > eng:
+		return docLangPair
+	case eng > pair:
+		return docLangEnglish
+	}
+	return ""
+}
+
+// corroborated requires the document as a whole to carry real evidence of the
+// pair language before the pass flips into it. A two-sentence document of
+// "Sim." / "Não." would otherwise reach 100% on almost nothing; a flip changes
+// every card in the document, so it has to be earned document-wide and not only
+// in proportion.
+func corroborated(contentText, p string) bool {
+	if p == "zh" {
+		han, _ := scriptCounts(contentText)
+		return han >= 8
+	}
+	return distinctMarkers(contentText, latinMarkers[p]) >= 3
+}
+
+// hasLangTest reports whether readsAsPairLang / sentenceLang know how to test
+// this pair at all. Kept beside the tests it describes so a new pair that adds
+// markers without adding itself here fails loudly in review rather than quietly
+// at runtime.
+func hasLangTest(p string) bool {
+	if p == "zh" {
+		return true
+	}
+	return len(latinMarkers[p]) > 0
+}
+
+// English function words, curated against the pair languages exactly as
+// `latinMarkers` is curated against English.
+//
+// Every word here is one a Portuguese, French or Spanish sentence has no reason
+// to contain. Deliberately absent, each a false English vote waiting to happen
+// in someone's own language: "on" and "son" (French), "as", "a", "o", "e", "no",
+// "os" (Portuguese), "no", "para", "sin" (Spanish), and "is"-alikes that are
+// really other languages' words. The list is short on purpose — it does not need
+// coverage, only a reliable vote in the sentences where the pair list is silent.
+var englishMarkers = words(
+	"the", "and", "is", "are", "was", "were", "be", "been", "being",
+	"of", "to", "that", "this", "these", "those", "with", "from", "for",
+	"have", "has", "had", "they", "them", "their", "there", "then", "than",
+	"what", "which", "when", "where", "why", "how", "who",
+	"will", "would", "should", "could", "can", "about", "because",
+	"into", "some", "such", "only", "very", "much", "many", "other",
+	"our", "your", "its", "it", "he", "she", "we", "you", "but", "not",
+	"just", "like", "also", "most", "over", "after", "before", "between",
+	"through", "said", "says", "get", "got", "make", "made", "know", "think",
+	"thing", "things", "time", "people", "here", "always", "never", "something",
+	"want", "need", "feel", "day", "today", "good", "really", "still", "even",
+)
+
+// normalizeDocLang folds a stored verdict into the two values the rest of the
+// code reasons about: the column holds "" for a document nothing has read yet,
+// and that means English, which is what every surface did before this phase.
+func normalizeDocLang(v string) string {
+	if strings.TrimSpace(v) == docLangPair {
+		return docLangPair
+	}
+	return docLangEnglish
+}

internal/suggestions/doclang_test.go

@@ -0,0 +1,395 @@
+package suggestions
+
+import (
+	"encoding/json"
+	"net/http"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/go-chi/chi/v5"
+
+	"gitea.parodia.dev/drwily/petal/internal/auth"
+	"gitea.parodia.dev/drwily/petal/internal/db"
+	"gitea.parodia.dev/drwily/petal/internal/llm"
+)
+
+// A monolingual document in either language has to be read as that language, and
+// the mixed cases in between are where the whole design lives: one quotation
+// must not move a document, and one leftover English line must not hold a
+// journal in English.
+func TestDocumentLangReadsWholeDocuments(t *testing.T) {
+	const ptJournal = "Hoje foi um dia muito bom. Eu gosto de escrever aqui todas as noites. " +
+		"A minha irmã também quer aprender. Não sei porque isso é tão difícil para mim."
+	const enEssay = "The weather was very cold this morning. I think that the bus was late again. " +
+		"She told me about the meeting, but I could not hear what they said."
+	const zhJournal = "今天天气很好。我和妹妹一起去公园散步。我们看到很多花。"
+
+	tests := []struct {
+		name     string
+		text     string
+		pairLang string
+		prev     string
+		want     string
+	}{
+		{"portuguese journal", ptJournal, "pt-PT", "", docLangPair},
+		{"english essay", enEssay, "pt-PT", "", docLangEnglish},
+		{"chinese journal", zhJournal, "zh", "", docLangPair},
+		{"english essay, zh writer", enEssay, "zh", "", docLangEnglish},
+
+		// One English sentence at the end of a Portuguese journal is the case that
+		// motivated the whole phase: the pass must stay in Portuguese.
+		{
+			"portuguese with one english line",
+			ptJournal + " I will write more tomorrow.",
+			"pt-PT", "", docLangPair,
+		},
+		// And the mirror: an English essay quoting a line of Portuguese is still an
+		// English essay.
+		{
+			"english quoting portuguese",
+			enEssay + " She wrote: \"Eu não sei o que dizer.\"",
+			"pt-PT", "", docLangEnglish,
+		},
+		// A pair Petal has no test for cannot flip anything. Saying English is what
+		// every surface did before this phase.
+		{"untested pair", ptJournal, "de", "", docLangEnglish},
+		// Nothing to go on holds the previous answer rather than resetting a
+		// journal because she cleared it to start again.
+		{"emptied portuguese journal", "", "pt-PT", docLangPair, docLangPair},
+		{"emptied english essay", "   \n  ", "pt-PT", docLangEnglish, docLangEnglish},
+		// Proportion, not presence: a couple of Portuguese words are not a
+		// Portuguese document even though readsAsPairLang would label that span.
+		{
+			"english with a portuguese phrase",
+			enEssay + " The sign said pão com manteiga.",
+			"pt-PT", "", docLangEnglish,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := documentLang(tc.text, tc.pairLang, tc.prev); got != tc.want {
+				t.Fatalf("documentLang = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+// The band, from both directions. A document sitting inside it keeps whatever it
+// was, and that is the point: without it, a bilingual paragraph would alternate
+// its cards' language every few keystrokes as she typed across the threshold.
+func TestDocumentLangHysteresis(t *testing.T) {
+	// Half and half: two Portuguese sentences, two English ones. Inside the band
+	// from either side.
+	const mixed = "Eu gosto muito de escrever aqui. A minha irmã não sabe porque é difícil. " +
+		"The weather was very cold this morning. I think that they said the same thing."
+
+	if got := documentLang(mixed, "pt-PT", docLangEnglish); got != docLangEnglish {
+		t.Fatalf("mixed document from english = %q, want it to stay %q", got, docLangEnglish)
+	}
+	if got := documentLang(mixed, "pt-PT", docLangPair); got != docLangPair {
+		t.Fatalf("mixed document from pair = %q, want it to stay %q", got, docLangPair)
+	}
+
+	// Above the upper threshold it flips regardless of where it came from; below
+	// the lower one it flips back regardless.
+	const mostlyPT = "Eu gosto muito de escrever aqui. A minha irmã não sabe porque é difícil. " +
+		"Hoje foi um dia bom para mim. Amanhã também quero escrever mais uma coisa. " +
+		"I think so too."
+	if got := documentLang(mostlyPT, "pt-PT", docLangEnglish); got != docLangPair {
+		t.Fatalf("mostly-portuguese from english = %q, want %q", got, docLangPair)
+	}
+	const mostlyEN = "The weather was very cold this morning. I think that they said the same thing. " +
+		"She could not hear what the other people were saying about it. Eu não sei."
+	if got := documentLang(mostlyEN, "pt-PT", docLangPair); got != docLangEnglish {
+		t.Fatalf("mostly-english from pair = %q, want %q", got, docLangEnglish)
+	}
+}
+
+// Corroboration: a proportion computed over almost nothing is not evidence. Two
+// bare words at 100% must not flip a document, because a flip rewrites every
+// card in it.
+func TestDocumentLangNeedsCorroboration(t *testing.T) {
+	if got := documentLang("Não. Eu.", "pt-PT", docLangEnglish); got != docLangEnglish {
+		t.Fatalf("two bare words flipped the document: %q", got)
+	}
+	if got := documentLang("我。", "zh", docLangEnglish); got != docLangEnglish {
+		t.Fatalf("two Han runes flipped the document: %q", got)
+	}
+}
+
+// The two language decisions are genuinely independent, and only the zh pair can
+// prove it today — it is the one pair that can be travelled in both directions.
+//
+// A Mandarin native practising English who writes Chinese wants Chinese
+// corrections explained in Chinese. An English native learning Chinese who writes
+// Chinese wants the same Chinese corrections explained in English. Same document,
+// same Correct, different Explain.
+func TestTargetSeparatesCorrectedFromExplained(t *testing.T) {
+	learningEn := targetFor("zh", auth.DirectionLearningEn, docLangPair)
+	if learningEn.Correct.Code != "zh" || learningEn.Explain.Code != "zh" {
+		t.Fatalf("learning_en on a Chinese document: correct=%s explain=%s", learningEn.Correct.Code, learningEn.Explain.Code)
+	}
+
+	learningPair := targetFor("zh", auth.DirectionLearningPair, docLangPair)
+	if learningPair.Correct.Code != "zh" {
+		t.Fatalf("learner direction changed what gets corrected: %s", learningPair.Correct.Code)
+	}
+	if learningPair.Explain.Code != "en" {
+		t.Fatalf("learner direction explained in %s, want English", learningPair.Explain.Code)
+	}
+
+	// An English document is the path every account is on today, in either
+	// direction: English corrections, English explanations, her language still on
+	// the Ask Petal and translate taps.
+	for _, dir := range []string{auth.DirectionLearningEn, auth.DirectionLearningPair} {
+		got := targetFor("zh", dir, docLangEnglish)
+		if got.Flipped() || got.Explain.Code != "en" {
+			t.Fatalf("english document with direction %s: %+v", dir, got)
+		}
+		if got.Pair.Code != "zh" {
+			t.Fatalf("english document lost the writer's pair: %+v", got)
+		}
+	}
+}
+
+// newDirectedServer seeds one writer on a given pair and direction, with a
+// document of her own. Like newPairServer, but the direction is the variable.
+func newDirectedServer(t *testing.T, client llm.LLMClient, pairLang, direction, text string) (http.Handler, string, *Handler) {
+	t.Helper()
+	database, err := db.Open(filepath.Join(t.TempDir(), "doclang.db"))
+	if err != nil {
+		t.Fatalf("open db: %v", err)
+	}
+	t.Cleanup(func() { database.Close() })
+
+	const userID = "writer-directed"
+	if _, err := database.Exec(
+		`INSERT INTO users (id, email, display_name, pair_lang, direction) VALUES (?, ?, ?, ?, ?)`,
+		userID, "d@example.com", "Writer", pairLang, direction,
+	); err != nil {
+		t.Fatalf("seed user: %v", err)
+	}
+
+	var docID string
+	if err := database.QueryRow(
+		`INSERT INTO documents (user_id, content_text) VALUES (?, ?) RETURNING id`,
+		userID, text,
+	).Scan(&docID); err != nil {
+		t.Fatalf("seed doc: %v", err)
+	}
+
+	h := New(database, client)
+	h.Limit = llm.NewRateLimiter(0)
+	h.VoiceLimit = llm.NewRateLimiter(0)
+	r := chi.NewRouter()
+	r.Route("/docs", func(dr chi.Router) { h.RegisterDocRoutes(dr) })
+	r.Mount("/suggestions", h.Routes())
+	return auth.Middleware(auth.StaticResolver(userID))(r), docID, h
+}
+
+const ptDocument = "Hoje foi um dia muito bom. Eu gosto de escrever aqui todas as noites. " +
+	"A minha irmã também quer aprender comigo. Não sei porque isso é tão difícil para mim."
+
+// End to end: a Portuguese document reaches the model as a Portuguese
+// checkpoint. This is the observed bug from 2026-07-28 — two pt-PT sentences
+// drew no cards at all, because Petal was reading them as bad English.
+func TestCheckpointFollowsTheDocumentLanguage(t *testing.T) {
+	client := &stubClient{response: `{"suggestions":[]}`}
+	srv, docID, _ := newDirectedServer(t, client, "pt-PT", auth.DirectionLearningEn, ptDocument)
+
+	if rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/check", ""); rec.Code != http.StatusOK {
+		t.Fatalf("check: code=%d body=%s", rec.Code, rec.Body)
+	}
+	if !strings.Contains(client.lastPrompt, "European Portuguese") {
+		t.Fatalf("checkpoint didn't follow the document into Portuguese:\n%s", client.lastPrompt)
+	}
+	if strings.Contains(client.lastPrompt, "second language") {
+		t.Fatalf("checkpoint kept the ESL framing on a Portuguese document:\n%s", client.lastPrompt)
+	}
+
+	// And the voice pass, which had no language argument at all before this phase.
+	if rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/voice", ""); rec.Code != http.StatusOK {
+		t.Fatalf("voice: code=%d body=%s", rec.Code, rec.Body)
+	}
+	if !strings.Contains(client.lastPrompt, "European Portuguese") {
+		t.Fatalf("voice pass didn't follow the document:\n%s", client.lastPrompt)
+	}
+}
+
+// The verdict is persisted, because hysteresis needs a yesterday.
+func TestDocumentLangIsRemembered(t *testing.T) {
+	client := &stubClient{response: `{"suggestions":[]}`}
+	srv, docID, h := newDirectedServer(t, client, "pt-PT", auth.DirectionLearningEn, ptDocument)
+
+	if rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/check", ""); rec.Code != http.StatusOK {
+		t.Fatalf("check: code=%d body=%s", rec.Code, rec.Body)
+	}
+	var stored string
+	if err := h.DB.QueryRow(`SELECT doc_lang FROM documents WHERE id = ?`, docID).Scan(&stored); err != nil {
+		t.Fatalf("read doc_lang: %v", err)
+	}
+	if stored != docLangPair {
+		t.Fatalf("doc_lang = %q, want %q", stored, docLangPair)
+	}
+}
+
+// A document that changes language re-opens every sentence. Without the verdict
+// in the chunk salt, the sentences she didn't touch would keep serving cards
+// written in the language the document no longer speaks.
+func TestLanguageFlipReopensCheckedSentences(t *testing.T) {
+	client := &stubClient{response: `{"suggestions":[]}`}
+	const enStart = "The weather was very cold this morning."
+	srv, docID, h := newDirectedServer(t, client, "pt-PT", auth.DirectionLearningEn, enStart)
+
+	if rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/check", ""); rec.Code != http.StatusOK {
+		t.Fatalf("first check: code=%d body=%s", rec.Code, rec.Body)
+	}
+	first := client.calls
+
+	// She rewrites the document in Portuguese, keeping the first sentence.
+	setDocText(t, h, docID, enStart+" "+ptDocument)
+	if rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/check", ""); rec.Code != http.StatusOK {
+		t.Fatalf("second check: code=%d body=%s", rec.Code, rec.Body)
+	}
+	if client.calls == first {
+		t.Fatal("the flipped document was never sent to the model")
+	}
+	if !strings.Contains(client.lastPrompt, "The weather was very cold") {
+		t.Fatalf("the already-checked sentence was not re-opened by the flip:\n%s", client.lastPrompt)
+	}
+}
+
+// The translate card, pointed the other way. She is writing her journal in
+// Portuguese and drops in the one English sentence she knows; Petal renders it
+// into Portuguese, and that card is a translation — not a correction to prose
+// that was never wrong.
+func TestEnglishSpanBecomesATranslateCardInAPortugueseDocument(t *testing.T) {
+	const english = "I want to say this but I don't know how to say it."
+	// The model volunteers "clarity", as it did for the zh case. Not consulted.
+	client := &stubClient{response: `{"suggestions":[
+		{"original":"` + english + `","replacement":"Eu quero dizer isto mas não sei como o dizer.","explanation":"Aqui está em português.","type":"clarity"}
+	]}`}
+	srv, docID, _ := newDirectedServer(t, client, "pt-PT", auth.DirectionLearningEn, ptDocument+" "+english)
+
+	var out []db.Suggestion
+	rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
+	if rec.Code != http.StatusOK {
+		t.Fatalf("check: code=%d body=%s", rec.Code, rec.Body)
+	}
+	if err := json.Unmarshal(rec.Body.Bytes(), &out); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(out) != 1 {
+		t.Fatalf("want 1 card, got %d: %+v", len(out), out)
+	}
+	if out[0].Type != db.SuggestionTypeTranslate {
+		t.Fatalf("card type = %q, want %q", out[0].Type, db.SuggestionTypeTranslate)
+	}
+}
+
+// And the half that keeps it honest: a genuine Portuguese correction in the same
+// document stays a correction. Reading the English-document test backwards would
+// have called this a translation, because every Portuguese sentence also "reads
+// as English" by that test's deliberately low bar.
+func TestPortugueseCorrectionKeepsItsTypeInAPortugueseDocument(t *testing.T) {
+	client := &stubClient{response: `{"suggestions":[
+		{"original":"Não sei porque isso é tão difícil para mim.","replacement":"Não sei porque isto é tão difícil para mim.","explanation":"Aqui usa-se isto.","type":"grammar"}
+	]}`}
+	srv, docID, _ := newDirectedServer(t, client, "pt-PT", auth.DirectionLearningEn, ptDocument)
+
+	var out []db.Suggestion
+	rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
+	if rec.Code != http.StatusOK {
+		t.Fatalf("check: code=%d body=%s", rec.Code, rec.Body)
+	}
+	if err := json.Unmarshal(rec.Body.Bytes(), &out); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(out) != 1 {
+		t.Fatalf("want 1 card, got %d: %+v", len(out), out)
+	}
+	if out[0].Type == db.SuggestionTypeTranslate {
+		t.Fatal("a Portuguese correction inside a Portuguese document was labelled a translation")
+	}
+}
+
+// setDocLang writes a document's language verdict directly, so a test of the
+// tap-through doesn't have to run a checkpoint through the same stub client to
+// get one.
+func setDocLang(t *testing.T, h *Handler, docID, lang string) {
+	t.Helper()
+	if _, err := h.DB.Exec(`UPDATE documents SET doc_lang = ? WHERE id = ?`, lang, docID); err != nil {
+		t.Fatalf("set doc_lang: %v", err)
+	}
+}
+
+// seedExplanation files one card carrying a given explanation and returns its
+// id — the shape the translate tap-through needs, where only the explanation and
+// the document it hangs off matter.
+func seedExplanation(t *testing.T, h *Handler, docID, explanation string) string {
+	t.Helper()
+	var sugID string
+	if err := h.DB.QueryRow(
+		`INSERT INTO suggestions (doc_id, original, replacement, explanation, type, from_pos, to_pos)
+		 VALUES (?, ?, ?, ?, ?, 0, 5) RETURNING id`,
+		docID, "isso", "isto", explanation, "grammar",
+	).Scan(&sugID); err != nil {
+		t.Fatalf("seed suggestion: %v", err)
+	}
+	return sugID
+}
+
+// The tap-through has to read the same decision the card was written under. On a
+// Portuguese document by a Portuguese writer the explanation already arrived in
+// Portuguese, and the old endpoint would have sent it to the model to be
+// rendered into Portuguese again.
+func TestTranslateSkipsWhenTheExplanationIsAlreadyHers(t *testing.T) {
+	client := &stubClient{response: "Não devia ser chamado."}
+	srv, docID, h := newDirectedServer(t, client, "pt-PT", auth.DirectionLearningEn, ptDocument)
+	setDocLang(t, h, docID, docLangPair)
+	sugID := seedExplanation(t, h, docID, "Aqui usa-se isto.")
+
+	rec := do(t, srv, http.MethodPost, "/suggestions/"+sugID+"/translate", "")
+	if rec.Code != http.StatusOK {
+		t.Fatalf("translate: code=%d body=%s", rec.Code, rec.Body)
+	}
+	var out translateResponse
+	if err := json.Unmarshal(rec.Body.Bytes(), &out); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if out.Translation != "" {
+		t.Fatalf("translation = %q, want empty: the bubble seeds from the explanation itself", out.Translation)
+	}
+	if client.calls != 0 {
+		t.Fatal("the model was asked to render Portuguese into Portuguese")
+	}
+}
+
+// The learner travelling the other way is the case that proves the endpoint
+// derives its destination rather than skipping whenever a document is flipped: a
+// native English speaker learning Chinese, writing Chinese, gets her
+// explanations in English — and the tap still has somewhere to go.
+func TestTranslateStillRendersForALearnersEnglishExplanation(t *testing.T) {
+	const zhDocument = "今天天气很好。我早上去公园散步。下午我在家里写作业。晚上我和朋友一起吃饭。"
+	client := &stubClient{response: "这里应该用这个。"}
+	srv, docID, h := newDirectedServer(t, client, "zh", auth.DirectionLearningPair, zhDocument)
+	setDocLang(t, h, docID, docLangPair)
+	sugID := seedExplanation(t, h, docID, "This measure word doesn't fit here.")
+
+	rec := do(t, srv, http.MethodPost, "/suggestions/"+sugID+"/translate", "")
+	if rec.Code != http.StatusOK {
+		t.Fatalf("translate: code=%d body=%s", rec.Code, rec.Body)
+	}
+	var out translateResponse
+	if err := json.Unmarshal(rec.Body.Bytes(), &out); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if out.Translation == "" {
+		t.Fatal("a learner's English explanation was left untranslated")
+	}
+	if !strings.Contains(client.lastPrompt, "Simplified Chinese") {
+		t.Fatalf("translate didn't render into the pair language:\n%s", client.lastPrompt)
+	}
+}

internal/suggestions/handlers.go

@@ -247,12 +247,45 @@ func (h *Handler) collocation(w http.ResponseWriter, r *http.Request) {
 }
 
 // pass is the signature shared by the grammar checkpoint and the voice pass:
-// given the document text, the document's tone and the writer's pair language it
-// returns the model's raw suggestions. The voice pass ignores both extras (see
-// llm.RunVoice) and the checkpoint ignores the language — only the collocation
-// coach writes a word of it — but one signature keeps runPass free of special
-// cases.
-type pass func(ctx context.Context, client llm.LLMClient, contentText, tone string, lang llm.Lang) ([]llm.RawSuggestion, error)
+// given the document text, the document's tone and the languages this document
+// is to be corrected and explained in, it returns the model's raw suggestions.
+// The voice pass ignores the tone (see llm.RunVoice) and the collocation coach
+// reads only the writer's pair language, but one signature keeps runPass free of
+// special cases.
+type pass func(ctx context.Context, client llm.LLMClient, contentText, tone string, t llm.Target) ([]llm.RawSuggestion, error)
+
+// targetFor resolves the two language decisions for one pass over one document.
+//
+// They read different state on purpose. What gets *corrected* follows the
+// document, because Portuguese prose wants Portuguese corrections. What language
+// the correction is *explained* in follows the writer — the half of her pair she
+// is not learning — because an explanation is teaching, and teaching lands in the
+// language she reads most easily. A native Portuguese speaker practising English
+// gets Portuguese explained in Portuguese; a native English speaker learning
+// French gets French explained in English. Neither is trapped: the other language
+// stays one tap away, in both directions.
+//
+// An English document keeps the pre-Phase-28 behaviour exactly — explained in
+// English, with her language on the Ask Petal / translate taps — which is the
+// path every account today is on.
+//
+// The direction lookup costs nothing today: `learnerPairs` is {"zh"}, so fr, es
+// and pt-PT accounts are all learning_en and their non-learned half *is* the pair
+// language. This rule therefore produces "explain in the document's language" for
+// every writer who currently exists. It is written out anyway to stop the
+// coincidence being baked into the prompts, the way "English is the language
+// being learned" was baked into pair_lang before migration 0016.
+func targetFor(pairLang, direction, docLang string) llm.Target {
+	pair := llm.LangFor(pairLang)
+	if normalizeDocLang(docLang) != docLangPair {
+		return llm.EnglishTarget(pair)
+	}
+	explain := pair
+	if direction == auth.DirectionLearningPair {
+		explain = llm.English
+	}
+	return llm.Target{Correct: pair, Explain: explain, Pair: pair}
+}
 
 // runPass is the shared body for both LLM passes. It loads the document text,
 // enforces the pass's per-document rate limit, runs the model, swaps in the
@@ -262,17 +295,19 @@ func (h *Handler) runPass(w http.ResponseWriter, r *http.Request, limiter *llm.R
 	docID := chi.URLParam(r, "id")
 	userID := auth.UserID(r.Context())
 
-	// The writer's pair language rides along with the document rather than in a
-	// second query: it is read from the same row-scoped lookup that already
-	// proves she owns this document.
-	var contentText, tone, pairLang string
+	// The writer's pair language and direction ride along with the document
+	// rather than in a second query: they are read from the same row-scoped
+	// lookup that already proves she owns this document. `doc_lang` is the
+	// previous language verdict, which the new one needs (hysteresis).
+	var contentText, tone, pairLang, direction, prevLang string
 	err := h.DB.QueryRow(
-		`SELECT d.content_text, d.tone, COALESCE(u.pair_lang, '')
+		`SELECT d.content_text, d.tone, d.doc_lang,
+		        COALESCE(u.pair_lang, ''), COALESCE(u.direction, '')
 		   FROM documents d
 		   JOIN users u ON u.id = d.user_id
 		  WHERE d.id = ? AND d.user_id = ?`,
 		docID, userID,
-	).Scan(&contentText, &tone, &pairLang)
+	).Scan(&contentText, &tone, &prevLang, &pairLang, &direction)
 	if errors.Is(err, sql.ErrNoRows) {
 		httputil.ErrorJSON(w, http.StatusNotFound, "document not found")
 		return
@@ -282,10 +317,22 @@ func (h *Handler) runPass(w http.ResponseWriter, r *http.Request, limiter *llm.R
 		return
 	}
 
+	// What language is this document in, and so what language should its cards be
+	// written in? Computed from the whole content_text — never from `askText`,
+	// which on a chunked pass is only the sentences that changed, and would put an
+	// English card in a Portuguese journal the moment she edits its one English
+	// line.
+	//
+	// Decided before the empty-document exit so every reconcile below is told the
+	// same verdict. An emptied document has nothing to go on and holds whatever it
+	// said last (see documentLang), which is what keeps a Portuguese journal
+	// Portuguese while she clears it to start the entry again.
+	docLang := documentLang(contentText, pairLang, prevLang)
+
 	// Nothing to analyze on an empty document — skip the LLM round-trip. The
 	// family's rows go with the text they were about.
 	if strings.TrimSpace(contentText) == "" {
-		if err := h.reconcilePending(docID, contentText, pairLang, nil, scope, nil, nil, false); err != nil {
+		if err := h.reconcilePending(docID, contentText, pairLang, docLang, nil, scope, nil, nil, false); err != nil {
 			httputil.ServerError(w, err)
 			return
 		}
@@ -298,16 +345,30 @@ func (h *Handler) runPass(w http.ResponseWriter, r *http.Request, limiter *llm.R
 		return
 	}
 
+	if docLang != normalizeDocLang(prevLang) {
+		if _, err := h.DB.Exec(
+			`UPDATE documents SET doc_lang = ? WHERE id = ? AND user_id = ?`,
+			docLang, docID, userID,
+		); err != nil {
+			httputil.ServerError(w, err)
+			return
+		}
+	}
+	target := targetFor(pairLang, direction, docLang)
+
 	// Decide what to ask about before spending anything: a chunked pass asks only
 	// about the sentences that changed since it last read the document, and when
 	// none did it doesn't call the model at all — nor consume its rate-limit slot,
 	// so the next real edit isn't throttled by a check that had nothing to do.
 	//
 	// Only a chunked pass consults that record, so only it needs the tone folded
-	// into a sentence's identity.
+	// into a sentence's identity — and, next to it, the language verdict. A
+	// document that flips language changes every sentence's identity, so its
+	// old-language cards are re-checked rather than left sitting there in a
+	// language the rest of the document no longer speaks.
 	salt := ""
 	if scope.chunked {
-		salt = tone
+		salt = tone + "\x00" + docLang
 	}
 	chunks := splitChunks(contentText, salt)
 	askText, fresh := contentText, chunks
@@ -321,7 +382,7 @@ func (h *Handler) runPass(w http.ResponseWriter, r *http.Request, limiter *llm.R
 		if len(changed) == 0 {
 			// Every sentence has already been read. Drop the rows whose sentence is
 			// gone, keep the rest exactly as they are, and answer immediately.
-			if err := h.reconcilePending(docID, contentText, pairLang, nil, scope, chunks, nil, false); err != nil {
+			if err := h.reconcilePending(docID, contentText, pairLang, docLang, nil, scope, chunks, nil, false); err != nil {
 				httputil.ServerError(w, err)
 				return
 			}
@@ -354,7 +415,7 @@ func (h *Handler) runPass(w http.ResponseWriter, r *http.Request, limiter *llm.R
 		return
 	}
 
-	raw, err := run(r.Context(), h.Client, askText, tone, llm.LangFor(pairLang))
+	raw, err := run(r.Context(), h.Client, askText, tone, target)
 	if err != nil {
 		// Allow ran before the model call, so a failed pass would otherwise hold
 		// the per-document slot for the full interval — stranding the frontend's
@@ -366,7 +427,7 @@ func (h *Handler) runPass(w http.ResponseWriter, r *http.Request, limiter *llm.R
 
 	// A whole-document pass re-read everything, so every one of its rows is up for
 	// re-proposal; a chunked pass only puts the sentences it asked about in play.
-	if err := h.reconcilePending(docID, contentText, pairLang, raw, scope, chunks, fresh, !scope.chunked); err != nil {
+	if err := h.reconcilePending(docID, contentText, pairLang, docLang, raw, scope, chunks, fresh, !scope.chunked); err != nil {
 		httputil.ServerError(w, err)
 		return
 	}
@@ -764,13 +825,13 @@ func (h *Handler) setStatus(w http.ResponseWriter, r *http.Request, status strin
 // hands over a reusable chunk, which is the only thing worth reviewing in a week.
 func (h *Handler) plant(id, userID string) {
 	var s db.Suggestion
-	var contentText string
+	var contentText, docLang string
 	err := h.DB.QueryRow(
-		`SELECT s.type, s.original, s.replacement, s.explanation, s.doc_id, d.content_text
+		`SELECT s.type, s.original, s.replacement, s.explanation, s.doc_id, d.content_text, d.doc_lang
 		   FROM suggestions s JOIN documents d ON d.id = s.doc_id
 		  WHERE s.id = ? AND d.user_id = ?`,
 		id, userID,
-	).Scan(&s.Type, &s.Original, &s.Replacement, &s.Explanation, &s.DocID, &contentText)
+	).Scan(&s.Type, &s.Original, &s.Replacement, &s.Explanation, &s.DocID, &contentText, &docLang)
 	if err != nil {
 		if !errors.Is(err, sql.ErrNoRows) {
 			log.Printf("suggestions: could not read %s for planting: %v", id, err)
@@ -789,6 +850,9 @@ func (h *Handler) plant(id, userID string) {
 		Meaning: s.Explanation,
 		Example: correctedSentence(contentText, s.Original, s.Replacement),
 		DocID:   &docID,
+		// The chunk is her own sentence, corrected — so it is in the document's
+		// language, whatever the collocation pass was framed in.
+		Lang: docLang,
 	}); err != nil {
 		log.Printf("suggestions: could not plant %s: %v", id, err)
 	}

internal/suggestions/language.go

@@ -28,16 +28,35 @@ import (
 //     common words a sentence in that language can hardly avoid and an English
 //     sentence has no reason to contain.
 
-// isTranslation reports whether this edit is her own language rendered into
-// English, rather than a correction to her English. Both halves must hold: the
-// quoted span reads as the pair language, and what Petal offers back reads as
-// English. The second half matters — a Chinese span rewritten into different
-// Chinese is something else entirely, and Petal has no business calling it a
-// translation.
-func isTranslation(original, replacement, pairLang string) bool {
+// isTranslation reports whether this edit is a rendering of one language into
+// the other, rather than a correction. Both halves must hold: the quoted span
+// reads as one language, and what Petal offers back reads as the other. The
+// second half matters — a Chinese span rewritten into different Chinese is
+// something else entirely, and Petal has no business calling it a translation.
+//
+// Which way it points follows the document (Phase 28). In an English document
+// the translate card is her language rendered into English — she reached for a
+// sentence she couldn't say yet, and Petal said it for her. In a document she
+// wrote in her own language the useful card is the mirror image: an English
+// sentence she dropped into her Portuguese, rendered into Portuguese. Asking the
+// English-document question there would label nothing, and the card would file
+// as a correction to prose that was never wrong.
+//
+// The flipped direction cannot be the same test read backwards. `readsAsEnglish`
+// is a low bar on purpose — Latin letters, not swamped by another script — which
+// every Portuguese sentence also clears, so using it on the *original* would
+// call every genuine Portuguese correction a translation. The flipped test
+// instead uses the sentence-level vote from doclang.go, where English has its
+// own marker list and has to out-evidence the pair language to win.
+func isTranslation(original, replacement, pairLang, docLang string) bool {
 	if strings.TrimSpace(original) == "" || strings.TrimSpace(replacement) == "" {
 		return false
 	}
+	if normalizeDocLang(docLang) == docLangPair {
+		p := normalizePairLang(pairLang)
+		return sentenceLang(original, p) == docLangEnglish &&
+			sentenceLang(replacement, p) == docLangPair
+	}
 	return readsAsPairLang(original, pairLang) && readsAsEnglish(replacement)
 }
 

internal/suggestions/language_test.go

@@ -3,6 +3,10 @@ package suggestions
 import "testing"
 
 // The flagship case, and the ones next to it that must NOT become translations.
+//
+// Every case here is an ENGLISH document — the path every account was on before
+// Phase 28 — so `docLang` is left at "". The mirror image lives in
+// TestIsTranslationInAPairLanguageDocument below.
 func TestIsTranslation(t *testing.T) {
 	cases := []struct {
 		name        string
@@ -134,8 +138,93 @@ func TestIsTranslation(t *testing.T) {
 
 	for _, c := range cases {
 		t.Run(c.name, func(t *testing.T) {
-			if got := isTranslation(c.original, c.replacement, c.pairLang); got != c.want {
-				t.Errorf("isTranslation(%q, %q, %q) = %v, want %v",
+			if got := isTranslation(c.original, c.replacement, c.pairLang, ""); got != c.want {
+				t.Errorf("isTranslation(%q, %q, %q, en) = %v, want %v",
+					c.original, c.replacement, c.pairLang, got, c.want)
+			}
+		})
+	}
+}
+
+// The mirror image (Phase 28): in a document she wrote in her own language, the
+// translate card is the English sentence rendered into her language — and the
+// English-document question, asked here, would label nothing.
+//
+// The case this file exists to pin is the third one: a genuine Portuguese
+// correction inside a Portuguese document. Reading the English-document test
+// backwards would call it a translation, because `readsAsEnglish` is a low bar
+// that Portuguese clears too. It has to stay a correction.
+func TestIsTranslationInAPairLanguageDocument(t *testing.T) {
+	cases := []struct {
+		name        string
+		original    string
+		replacement string
+		pairLang    string
+		want        bool
+	}{
+		{
+			name:        "English sentence rendered into Portuguese",
+			original:    "I want to say this but I don't know how to say it.",
+			replacement: "Eu quero dizer isso mas não sei como o dizer.",
+			pairLang:    "pt-PT",
+			want:        true,
+		},
+		{
+			name:        "English sentence rendered into Chinese",
+			original:    "I don't know how to say this in Chinese.",
+			replacement: "我不知道这句话用中文怎么说。",
+			pairLang:    "zh",
+			want:        true,
+		},
+		{
+			// The one that matters. Portuguese in, Portuguese out, inside a
+			// Portuguese document: a correction, and nothing else.
+			name:        "Portuguese corrected as Portuguese",
+			original:    "Eu quero dizer isso mas não sei como.",
+			replacement: "Eu quero dizer isto mas não sei como.",
+			pairLang:    "pt-PT",
+			want:        false,
+		},
+		{
+			// And its Chinese twin, which the script test already caught.
+			name:        "Chinese corrected as Chinese",
+			original:    "我想说这句话",
+			replacement: "我要说这句话",
+			pairLang:    "zh",
+			want:        false,
+		},
+		{
+			// The old direction, asked in the new document. She quoted English in
+			// her Portuguese and Petal rendered it into Portuguese — which IS a
+			// translation, and is the case above. This is its reverse: Portuguese
+			// out of an English document that isn't one. No label.
+			name:        "Portuguese rendered into English is not this document's translation",
+			original:    "Eu quero dizer isso mas não sei como.",
+			replacement: "I want to say this but I don't know how.",
+			pairLang:    "pt-PT",
+			want:        false,
+		},
+		{
+			// English prose without enough evidence to vote. Silence, not a guess.
+			name:        "too short to read as English",
+			original:    "OK",
+			replacement: "Está bem, muito obrigado.",
+			pairLang:    "pt-PT",
+			want:        false,
+		},
+		{
+			name:        "unknown pair language declines in both directions",
+			original:    "I don't know how to say that.",
+			replacement: "Ich weiß nicht wie man das sagt.",
+			pairLang:    "de",
+			want:        false,
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			if got := isTranslation(c.original, c.replacement, c.pairLang, docLangPair); got != c.want {
+				t.Errorf("isTranslation(%q, %q, %q, pair) = %v, want %v",
 					c.original, c.replacement, c.pairLang, got, c.want)
 			}
 		})

internal/suggestions/plant_test.go

@@ -184,3 +184,60 @@ func TestCorrectedSentence(t *testing.T) {
 		t.Errorf("unterminated doc: got %q", got)
 	}
 }
+
+// TestPlantedPhraseCarriesTheDocumentLanguage: a chunk planted out of a document
+// written in her own language is a card in that language. The collocation pass
+// itself deliberately did not flip in Phase 28 — its prompt is per-language
+// knowledge, not framing — but the phrase it hands over is still lifted from her
+// prose, so the card has to know what language that prose was in or the garden
+// will read it aloud in the wrong voice.
+func TestPlantedPhraseCarriesTheDocumentLanguage(t *testing.T) {
+	srv, docID, h := newTestServer(t, &stubClient{})
+	if _, err := h.DB.Exec(`UPDATE documents SET doc_lang = 'pair' WHERE id = ?`, docID); err != nil {
+		t.Fatalf("set doc_lang: %v", err)
+	}
+	id := seedSuggestion(t, h, docID,
+		"Ontem foi difícil. Tive de tomar uma decisão sobre o trabalho.",
+		db.SuggestionTypeCollocation, "tomar uma decisão", "tomar uma decisão",
+		"Em português diz-se “tomar” uma decisão.")
+
+	if rec := do(t, srv, http.MethodPost, "/suggestions/"+id+"/accept", ""); rec.Code != http.StatusNoContent {
+		t.Fatalf("accept: got %d, want 204", rec.Code)
+	}
+
+	var lang string
+	if err := h.DB.QueryRow(
+		`SELECT lang FROM vocab_words WHERE user_id = ? AND word = ?`,
+		db.LocalUserID, "tomar uma decisão",
+	).Scan(&lang); err != nil {
+		t.Fatalf("read planted card: %v", err)
+	}
+	if lang != "pair" {
+		t.Fatalf("planted card lang = %q, want pair", lang)
+	}
+}
+
+// TestPlantedPhraseOnAnEnglishDocumentIsUnchanged is the other half, and the one
+// every account is on today: an English document plants an English card, and the
+// empty backfill and 'en' both mean that.
+func TestPlantedPhraseOnAnEnglishDocumentIsUnchanged(t *testing.T) {
+	srv, docID, h := newTestServer(t, &stubClient{})
+	id := seedSuggestion(t, h, docID, "I had to do a decision about the job.",
+		db.SuggestionTypeCollocation, "do a decision", "make a decision",
+		"English pairs “make” with “decision”.")
+
+	if rec := do(t, srv, http.MethodPost, "/suggestions/"+id+"/accept", ""); rec.Code != http.StatusNoContent {
+		t.Fatalf("accept: got %d, want 204", rec.Code)
+	}
+
+	var lang string
+	if err := h.DB.QueryRow(
+		`SELECT lang FROM vocab_words WHERE user_id = ? AND word = ?`,
+		db.LocalUserID, "make a decision",
+	).Scan(&lang); err != nil {
+		t.Fatalf("read planted card: %v", err)
+	}
+	if lang == "pair" {
+		t.Fatalf("planted card lang = %q on an English document, want en or empty", lang)
+	}
+}

internal/suggestions/reconcile.go

@@ -131,10 +131,12 @@ func reposition(tx *sql.Tx, row pendingRow, from, to int, chunkHash string) erro
 // collocation coach — where every row is up for re-proposal because the model
 // just re-read everything.
 //
-// `pairLang` is the writer's own language, needed only to type a finding that
-// turns out to be her language rendered into English (see language.go).
+// `pairLang` is the writer's own language and `docLang` this document's language
+// verdict; between them they type a finding that turns out to be one language
+// rendered into the other, in whichever direction this document makes useful
+// (see language.go).
 func (h *Handler) reconcilePending(
-	docID, contentText, pairLang string,
+	docID, contentText, pairLang, docLang string,
 	raw []llm.RawSuggestion,
 	scope pendingScope,
 	chunks, fresh []chunk,
@@ -235,7 +237,7 @@ func (h *Handler) reconcilePending(
 		typ := scope.forceType
 		if typ == "" {
 			typ = normalizeType(s.Type)
-			if isTranslation(s.Original, s.Replacement, pairLang) {
+			if isTranslation(s.Original, s.Replacement, pairLang, docLang) {
 				typ = db.SuggestionTypeTranslate
 			}
 		}

internal/suggestions/translate.go

@@ -17,23 +17,37 @@ type translateResponse struct {
 	Translation string `json:"translation"`
 }
 
-// translate renders a suggestion's English explanation into Simplified Chinese
-// for the Ask Petal bubble, so the ESL reader sees the "why" in her first
-// language instead of a second copy of the same English text. The explanation is
-// loaded server-side from the suggestion id (scoped to the local user) and never
-// trusted from the client, mirroring chat (spec Note #10).
+// translate renders a suggestion's explanation into the other half of the
+// writer's pair for the Ask Petal bubble, so she sees the "why" in the language
+// she reads most easily instead of a second copy of the same text. The
+// explanation is loaded server-side from the suggestion id (scoped to the
+// caller) and never trusted from the client, mirroring chat (spec Note #10).
+//
+// Which language it renders into cannot be assumed (Phase 28). Before that phase
+// every explanation was English and every rendering went into her language, so
+// "the pair language" was a safe constant. Now the explanation's language is a
+// decision — `targetFor`, from the document's verdict and her direction — and
+// this endpoint has to read the same decision back, or it round-trips Portuguese
+// into Portuguese and calls it a translation.
+//
+// So: render into whichever half the explanation is NOT already in, and when the
+// explanation already arrived in the language this bubble exists to reach her
+// in, skip the model call and answer "". The client seeds the bubble with the
+// explanation itself when the translation comes back empty, which is exactly
+// right — there is nothing to add.
 func (h *Handler) translate(w http.ResponseWriter, r *http.Request) {
 	sugID := chi.URLParam(r, "id")
 
-	var explanation, pairLang string
+	var explanation, pairLang, direction, docLang string
 	err := h.DB.QueryRow(
-		`SELECT s.explanation, COALESCE(u.pair_lang, '')
+		`SELECT s.explanation, COALESCE(u.pair_lang, ''),
+		        COALESCE(u.direction, ''), d.doc_lang
 		   FROM suggestions s
 		   JOIN documents d ON d.id = s.doc_id
 		   JOIN users u ON u.id = d.user_id
 		  WHERE s.id = ? AND d.user_id = ?`,
 		sugID, auth.UserID(r.Context()),
-	).Scan(&explanation, &pairLang)
+	).Scan(&explanation, &pairLang, &direction, &docLang)
 	if errors.Is(err, sql.ErrNoRows) {
 		httputil.ErrorJSON(w, http.StatusNotFound, "suggestion not found")
 		return
@@ -49,7 +63,21 @@ func (h *Handler) translate(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	out, err := llm.RunTranslate(r.Context(), h.Client, explanation, llm.LangFor(pairLang))
+	// The explanation's own language, recovered from the same rule that chose it
+	// when the card was written. A card written before this phase — or on a
+	// document whose verdict has since flipped — is read as whatever the rule says
+	// today; the alternative is a language column on every suggestion row, and the
+	// cost of being wrong is one bubble seeded in the language it was already in.
+	target := targetFor(pairLang, direction, docLang)
+	if target.Explain.Code == target.Pair.Code {
+		// Already in her language. The other half is English — the language she is
+		// practising — and an unasked-for English rendering of an explanation she
+		// can already read is not a seed, it's noise.
+		httputil.WriteJSON(w, http.StatusOK, translateResponse{Translation: ""})
+		return
+	}
+
+	out, err := llm.RunTranslate(r.Context(), h.Client, explanation, target.Pair)
 	if err != nil {
 		httputil.UpstreamError(w, "translate", err)
 		return

internal/vocab/handlers.go

@@ -26,6 +26,11 @@ type Word struct {
 	Phonetic   string  `json:"phonetic"`
 	Example    string  `json:"example"`
 	DocID      *string `json:"doc_id"`
+	// Lang is '' | 'en' | 'pair' — the language of the document the word was met
+	// in (see migration 0018). '' reads as English, like everywhere else this
+	// vocabulary appears. The client needs it to pick a read-aloud voice: "comum"
+	// is unguessable from its letters, so the card has to carry the answer.
+	Lang         string     `json:"lang"`
 	DueAt        time.Time  `json:"due_at"`
 	IntervalDays int        `json:"interval_days"`
 	Ease         float64    `json:"ease"`
@@ -54,7 +59,7 @@ func (h *Handler) Routes() chi.Router {
 	return r
 }
 
-const vocabColumns = `id, word, gloss, definition, phonetic, example, doc_id,
+const vocabColumns = `id, word, gloss, definition, phonetic, example, doc_id, lang,
 	due_at, interval_days, ease, reps, lapses, last_reviewed, created_at`
 
 func scanWord(s interface {
@@ -62,7 +67,7 @@ func scanWord(s interface {
 }) (Word, error) {
 	var w Word
 	err := s.Scan(
-		&w.ID, &w.Word, &w.Gloss, &w.Definition, &w.Phonetic, &w.Example, &w.DocID,
+		&w.ID, &w.Word, &w.Gloss, &w.Definition, &w.Phonetic, &w.Example, &w.DocID, &w.Lang,
 		&w.DueAt, &w.IntervalDays, &w.Ease, &w.Reps, &w.Lapses, &w.LastReviewed, &w.CreatedAt,
 	)
 	return w, err
@@ -165,15 +170,22 @@ func (h *Handler) capture(w http.ResponseWriter, r *http.Request) {
 	// would hit the foreign key and leak a raw "FOREIGN KEY constraint" 500
 	// instead of a clean 400 (and, once auth lands, would let a word be attached
 	// to another user's document).
+	//
+	// The same row-scoped lookup answers what language the card is in: a word is
+	// met inside a document, so the document's verdict is the word's language.
+	// Asking the document rather than trusting a `lang` in the request body is
+	// the same choice `runPass` makes — the client never gets to name a language
+	// the server can already read. A word with no document is '', which reads as
+	// English.
+	lang := ""
 	if req.DocID != nil {
 		if strings.TrimSpace(*req.DocID) == "" {
 			req.DocID = nil
 		} else {
-			var ok int
 			err := h.DB.QueryRow(
-				`SELECT 1 FROM documents WHERE id = ? AND user_id = ?`,
+				`SELECT doc_lang FROM documents WHERE id = ? AND user_id = ?`,
 				*req.DocID, userID,
-			).Scan(&ok)
+			).Scan(&lang)
 			if errors.Is(err, sql.ErrNoRows) {
 				httputil.ErrorJSON(w, http.StatusBadRequest, "unknown doc_id")
 				return
@@ -189,15 +201,19 @@ func (h *Handler) capture(w http.ResponseWriter, r *http.Request) {
 	// schedule (due_at/reps/interval/ease) alone so re-looking-up a word never
 	// resets its progress.
 	_, err := h.DB.Exec(
-		`INSERT INTO vocab_words (user_id, word, gloss, definition, phonetic, example, doc_id, due_at, interval_days)
-		 VALUES (?, ?, ?, ?, ?, ?, ?, datetime('now', '+1 day'), 1)
+		`INSERT INTO vocab_words (user_id, word, gloss, definition, phonetic, example, doc_id, lang, due_at, interval_days)
+		 VALUES (?, ?, ?, ?, ?, ?, ?, ?, datetime('now', '+1 day'), 1)
 		 ON CONFLICT(user_id, word) DO UPDATE SET
 		   gloss      = excluded.gloss,
 		   definition = excluded.definition,
 		   phonetic   = excluded.phonetic,
 		   example    = CASE WHEN excluded.example != '' THEN excluded.example ELSE vocab_words.example END,
-		   doc_id     = COALESCE(excluded.doc_id, vocab_words.doc_id)`,
-		userID, word, req.Gloss, req.Definition, req.Phonetic, req.Example, req.DocID,
+		   doc_id     = COALESCE(excluded.doc_id, vocab_words.doc_id),
+		   -- lang travels with doc_id, and for the same reason: it is the new
+		   -- context or it is nothing. A lookup made outside any document must
+		   -- not relabel a card that was captured inside one.
+		   lang       = CASE WHEN excluded.doc_id IS NOT NULL THEN excluded.lang ELSE vocab_words.lang END`,
+		userID, word, req.Gloss, req.Definition, req.Phonetic, req.Example, req.DocID, lang,
 	)
 	if err != nil {
 		httputil.ServerError(w, err)

internal/vocab/handlers_test.go

@@ -253,3 +253,89 @@ func TestDocLinkSurvivesDocDelete(t *testing.T) {
 		t.Fatalf("doc_id should be nulled after doc delete, got %v", *all[0].DocID)
 	}
 }
+
+// TestCaptureTakesLanguageFromItsDocument is the garden's half of Phase 28: a
+// word met inside a document written in her own language is a card in that
+// language, and the server reads that off the document rather than being told.
+//
+// The three cases are the three the client can actually produce: a lookup inside
+// a flipped document, a lookup inside an English one, and a lookup with no
+// document at all (the search box) — the last of which is '', which reads as
+// English everywhere this value is used.
+func TestCaptureTakesLanguageFromItsDocument(t *testing.T) {
+	srv, database := newTestServer(t)
+
+	seed := func(lang string) string {
+		t.Helper()
+		var id string
+		if err := database.QueryRow(
+			`INSERT INTO documents (user_id, content_text, doc_lang) VALUES (?, 'hi', ?) RETURNING id`,
+			db.LocalUserID, lang,
+		).Scan(&id); err != nil {
+			t.Fatalf("seed doc: %v", err)
+		}
+		return id
+	}
+	pairDoc, enDoc := seed("pair"), seed("en")
+
+	capture := func(word, body string) Word {
+		t.Helper()
+		rec := do(t, srv, http.MethodPost, "/vocab", body)
+		if rec.Code != http.StatusCreated {
+			t.Fatalf("capture %s: code=%d body=%s", word, rec.Code, rec.Body)
+		}
+		var w Word
+		if err := json.Unmarshal(rec.Body.Bytes(), &w); err != nil {
+			t.Fatalf("decode %s: %v", word, err)
+		}
+		return w
+	}
+
+	if got := capture("comum", `{"word":"comum","doc_id":"`+pairDoc+`"}`); got.Lang != "pair" {
+		t.Fatalf("word from a flipped document: lang=%q, want pair", got.Lang)
+	}
+	if got := capture("reception", `{"word":"reception","doc_id":"`+enDoc+`"}`); got.Lang != "en" {
+		t.Fatalf("word from an English document: lang=%q, want en", got.Lang)
+	}
+	if got := capture("orphan", `{"word":"orphan"}`); got.Lang != "" {
+		t.Fatalf("word with no document: lang=%q, want empty", got.Lang)
+	}
+
+	// A client that names a language is ignored: the document is the authority,
+	// the same way runPass never lets the request pick its own target.
+	if got := capture("comum", `{"word":"comum","lang":"en","doc_id":"`+pairDoc+`"}`); got.Lang != "pair" {
+		t.Fatalf("client-supplied lang should not win: lang=%q, want pair", got.Lang)
+	}
+}
+
+// TestRecaptureOutsideADocumentKeepsItsLanguage pins the one asymmetry in the
+// upsert. Re-looking-up a word refreshes its context, but a lookup made with no
+// document carries no verdict — and relabelling a Portuguese card English
+// because she checked the word again from the search box would silently move it
+// to the wrong voice. lang travels with doc_id, or it doesn't travel.
+func TestRecaptureOutsideADocumentKeepsItsLanguage(t *testing.T) {
+	srv, database := newTestServer(t)
+	var docID string
+	if err := database.QueryRow(
+		`INSERT INTO documents (user_id, content_text, doc_lang) VALUES (?, 'olá', 'pair') RETURNING id`,
+		db.LocalUserID,
+	).Scan(&docID); err != nil {
+		t.Fatalf("seed doc: %v", err)
+	}
+	if rec := do(t, srv, http.MethodPost, "/vocab",
+		`{"word":"saudade","gloss":"","doc_id":"`+docID+`"}`); rec.Code != http.StatusCreated {
+		t.Fatalf("capture: code=%d body=%s", rec.Code, rec.Body)
+	}
+	rec := do(t, srv, http.MethodPost, "/vocab", `{"word":"saudade","gloss":"longing"}`)
+	if rec.Code != http.StatusCreated {
+		t.Fatalf("recapture: code=%d body=%s", rec.Code, rec.Body)
+	}
+	var w Word
+	_ = json.Unmarshal(rec.Body.Bytes(), &w)
+	if w.Lang != "pair" {
+		t.Fatalf("recapture outside a document: lang=%q, want pair held", w.Lang)
+	}
+	if w.Gloss != "longing" {
+		t.Fatalf("recapture should still refresh the gloss, got %q", w.Gloss)
+	}
+}

internal/vocab/plant.go

@@ -28,6 +28,10 @@ type Phrase struct {
 	Meaning string  // why it's better — the suggestion's explanation
 	Example string  // the sentence she met it in, already corrected
 	DocID   *string // where, so "where did I see this?" stays one tap
+	// Lang is the document's verdict ('' | 'en' | 'pair'), because the chunk is
+	// lifted out of her own prose and is therefore in whatever language that
+	// prose is. See migration 0018.
+	Lang string
 }
 
 // Phrase-card caps. A collocation is a short chunk; anything longer is a
@@ -84,13 +88,14 @@ func Plant(ex Execer, userID string, p Phrase) (bool, error) {
 		return false, nil
 	}
 	res, err := ex.Exec(
-		`INSERT INTO vocab_words (user_id, word, gloss, definition, phonetic, example, doc_id, due_at, interval_days)
-		 VALUES (?, ?, '', ?, '', ?, ?, datetime('now', '+1 day'), 1)
+		`INSERT INTO vocab_words (user_id, word, gloss, definition, phonetic, example, doc_id, lang, due_at, interval_days)
+		 VALUES (?, ?, '', ?, '', ?, ?, ?, datetime('now', '+1 day'), 1)
 		 ON CONFLICT(user_id, word) DO NOTHING`,
 		userID, key,
 		clamp(strings.TrimSpace(p.Meaning), maxDefinitionLen),
 		clamp(strings.TrimSpace(p.Example), maxExampleLen),
 		p.DocID,
+		p.Lang,
 	)
 	if err != nil {
 		return false, err

web/src/App.tsx

@@ -615,6 +615,7 @@ export default function App() {
                   <EditorCore
                     key={`${currentDoc.id}:${editorEpoch}`}
                     docId={currentDoc.id}
+                    docLang={currentDoc.doc_lang}
                     initialContent={currentDoc.content}
                     onChange={handleEditorChange}
                     segmenter={segmenter}

web/src/api/client.ts

@@ -44,8 +44,18 @@ export interface Document {
   // When true, this document's automatic snapshots are never pruned, so its
   // full writing trail survives as authorship evidence (see the passport).
   preserve_history: boolean
+  // Which language this document is written in, as decided server-side by the
+  // checkpoint pass: '' | 'en' | 'pair' ('' reads as English). Read-only — the
+  // editor never sends it. It is here so read-aloud can use the right voice on a
+  // document written in her own language.
+  doc_lang: DocLang
 }
 
+// The document-language verdict, shared by documents and garden cards. 'pair'
+// names the writer's own language rather than a language code, so changing her
+// pair re-reads her documents instead of stranding a stale name on them.
+export type DocLang = '' | 'en' | 'pair'
+
 // Fields the editor sends on auto-save. All optional so a rename can send title
 // alone; the editor sends the full set.
 export interface DocUpdate {
@@ -125,6 +135,9 @@ export interface VocabWord {
   phonetic: string
   example: string
   doc_id: string | null
+  // The language of the document this word was met in — the card's own language
+  // (migration 0018). Read-aloud needs it: "comum" is unguessable from letters.
+  lang: DocLang
   due_at: string
   interval_days: number
   ease: number

web/src/audio/speech.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest'
-import { nativeLang, speak, stopSpeech } from './speech'
+import { docLang, nativeLang, speak, stopSpeech } from './speech'
 import { resetPackForTests, setPackLang } from '../i18n'
 
 // Read-aloud has two jobs beyond "make a sound": ask for the right pace, and ask
@@ -85,3 +85,33 @@ describe('nativeLang', () => {
     expect(bodies.at(-1)).toMatchObject({ text: 'chat', lang: 'fr-FR' })
   })
 })
+
+describe('docLang', () => {
+  // The document's verdict is the answer for a passage lifted out of it, because
+  // for a Latin pair there is no other answer available: an English sentence and
+  // a Portuguese one are the same letters.
+  it('reads a flipped document in her own language', () => {
+    setPackLang('pt-PT')
+    expect(docLang('Ontem foi difícil.', 'pair')).toBe('pt-PT')
+  })
+
+  it('reads an English document in English, and treats the backfill as English', () => {
+    setPackLang('pt-PT')
+    expect(docLang('Yesterday was hard.', 'en')).toBe('en-US')
+    expect(docLang('Yesterday was hard.', '')).toBe('en-US')
+  })
+
+  it('lets the script win over the verdict, so quoted Chinese is never spelled out', () => {
+    // An English document quoting Chinese is 'en' by verdict, and the English
+    // voice reads Han characters one "Chinese letter" at a time — the one
+    // failure worse than silence.
+    setPackLang('zh')
+    expect(docLang('你好世界', 'en')).toBe('zh-CN')
+  })
+
+  it('is what the editor selection and the garden card ask for', () => {
+    setPackLang('fr')
+    speak('Le chat dort.', docLang('Le chat dort.', 'pair'))
+    expect(bodies.at(-1)).toMatchObject({ text: 'Le chat dort.', lang: 'fr-FR' })
+  })
+})

web/src/audio/speech.ts

@@ -90,6 +90,24 @@ export function nativeLang(): string {
   return pack().locale
 }
 
+// docLang turns a document-language verdict ('' | 'en' | 'pair', decided
+// server-side — see internal/suggestions/doclang.go) into a locale for a passage
+// taken out of that document. It is what the editor's read-aloud and the garden's
+// review card ask instead of guessing.
+//
+// The script test still wins, and that is not redundant with the verdict. A
+// Chinese sentence quoted inside an English document is 'en' by verdict and
+// still has to be read by the Chinese voice: the English voice spells Han
+// characters out one "Chinese letter" at a time, which is the one failure loud
+// enough to be worse than no audio. In the other direction there is nothing to
+// test — an English sentence inside Portuguese prose looks exactly like the
+// Portuguese around it — so the document's verdict is the only answer available,
+// and it is the answer this phase decided on.
+export function docLang(text: string, verdict: string): string {
+  if (CJK.test(text)) return 'zh-CN'
+  return verdict === 'pair' ? pack().locale : 'en-US'
+}
+
 // speak reads `text` aloud, cancelling anything already in flight so rapid taps
 // don't queue up. `lang` defaults to a guess from the text (Chinese vs English)
 // so callers can just pass the selection; pass an explicit locale to override.

web/src/components/Editor/EditorCore.tsx

@@ -33,8 +33,8 @@ import { Composition } from './Composition'
 import { RewritePreview, type RewriteStatus } from './RewritePreview'
 import { planBatch } from './acceptBatch'
 import { entryId, idAfterRemoval, stepId, type Direction, type Span } from './triage'
-import { api, type Suggestion, type SuggestionType, type WordInfo } from '../../api/client'
-import { speak, speechSupported } from '../../audio/speech'
+import { api, type DocLang, type Suggestion, type SuggestionType, type WordInfo } from '../../api/client'
+import { docLang as docLocale, speak, speechSupported } from '../../audio/speech'
 import type { SpellChecker } from '../../hooks/useSpellChecker'
 import { fromIME } from '../../lib/ime'
 import type { Segmenter } from '../../lib/segment'
@@ -65,6 +65,13 @@ export interface EditorChange {
 interface Props {
   // Changing docId reloads the editor with that document's content.
   docId: string
+  // Which language this document is written in, as the server decided it
+  // ('' | 'en' | 'pair'). Read-aloud is the only thing here that reads it: a
+  // Portuguese selection has to be read by the Portuguese voice, and nothing in
+  // the letters says so. Updated by the server on save, so it trails a language
+  // flip by one auto-save — a passage read in the old voice once is the whole
+  // cost of not blocking the editor on a check.
+  docLang: DocLang
   initialContent: string
   onChange: (change: EditorChange) => void
   // LLM suggestions to highlight; accept/dismiss notify the parent for the API
@@ -255,6 +262,7 @@ interface RewriteState {
 // decoration layer. Hovering a highlight opens its SuggestionCard.
 export function EditorCore({
   docId,
+  docLang,
   initialContent,
   onChange,
   suggestions,
@@ -1532,8 +1540,10 @@ export function EditorCore({
           <SelectionBubble
             style={{ top: selection.top, left: selection.left, transform: 'translateY(calc(-100% - 8px))' }}
             onRewrite={handleRewrite}
-            onSpeak={speechSupported() ? () => speak(selection.text) : null}
-            onSpeakSlow={speechSupported() ? () => speak(selection.text, undefined, true) : null}
+            onSpeak={speechSupported() ? () => speak(selection.text, docLocale(selection.text, docLang)) : null}
+            onSpeakSlow={
+              speechSupported() ? () => speak(selection.text, docLocale(selection.text, docLang), true) : null
+            }
           />
         )}
         {rewrite && (

web/src/components/Garden/GardenPanel.tsx

@@ -1,6 +1,6 @@
 import { useCallback, useEffect, useMemo, useState } from 'react'
 import { api, type VocabGrade, type VocabWord } from '../../api/client'
-import { speak, speechSupported, stopSpeech } from '../../audio/speech'
+import { docLang as docLocale, speak, speechSupported, stopSpeech } from '../../audio/speech'
 import { useFocusTrap } from '../../hooks/useFocusTrap'
 import { usePack, type Line } from '../../i18n'
 import { JournalView } from './JournalView'
@@ -314,7 +314,21 @@ function GardenView({
                       {blossom(w.reps)}
                     </span>
                     <span className="flex min-w-0 flex-1 flex-col">
+                      <span className="flex min-w-0 items-center gap-1.5">
                         <span className="truncate text-sm font-bold text-plum">{w.word}</span>
+                        {/* Only a card in her own language is marked. An English
+                            garden with a badge on every blossom would be a
+                            garden with no badges at all; the marker exists so
+                            the rare Portuguese word is legible among them. */}
+                        {w.lang === 'pair' && (
+                          <span
+                            className="shrink-0 rounded-full px-1.5 py-0.5 text-[9px] font-bold lowercase"
+                            style={{ background: 'var(--color-surface-alt)', color: 'var(--color-muted)' }}
+                          >
+                            {t.nativeName}
+                          </span>
+                        )}
+                      </span>
                       {(w.gloss || w.definition) && (
                         <span
                           className="truncate text-xs"
@@ -354,7 +368,7 @@ function GardenView({
                         {speechSupported() && (
                           <button
                             type="button"
-                            onClick={() => speak(w.word)}
+                            onClick={() => speak(w.word, docLocale(w.word, w.lang))}
                             className="rounded-full px-2.5 py-1 text-xs font-semibold"
                             style={{ background: 'var(--color-surface-alt)' }}
                           >
@@ -477,7 +491,7 @@ function ReviewSession({
                 <>
                   <button
                     type="button"
-                    onClick={() => speak(card.word)}
+                    onClick={() => speak(card.word, docLocale(card.word, card.lang))}
                     aria-label={`Pronounce ${card.word}`}
                     className="flex h-6 w-6 items-center justify-center rounded-full text-xs"
                     style={{ background: 'var(--color-surface)' }}
@@ -488,7 +502,7 @@ function ReviewSession({
                       worth hearing stretched out. */}
                   <button
                     type="button"
-                    onClick={() => speak(card.word, undefined, true)}
+                    onClick={() => speak(card.word, docLocale(card.word, card.lang), true)}
                     aria-label={`Pronounce ${card.word} slowly`}
                     title={t.garden.readSlowly}
                     className="flex h-6 w-6 items-center justify-center rounded-full text-xs"