diff --git a/apps/web/src/lib/utils/accept-language.test.ts b/apps/web/src/lib/utils/accept-language.test.ts
index 0d54588..66d8186 100644
--- a/apps/web/src/lib/utils/accept-language.test.ts
+++ b/apps/web/src/lib/utils/accept-language.test.ts
@@ -51,4 +51,28 @@ describe('detectLanguage (Accept-Language style strings)', () => {
 	it('AL-12: unknown language (fr) falls back to en', () => {
 		expect(detectLanguage('fr-FR')).toBe('en');
 	});
+
+	// ─── Fase 20.4.1 — Euskera (eu) support ───────────────────────────────
+	// Mirrors the existing es-track tests. Coverage matches the plan's
+	// LANG-U-EU-01..03 identifiers.
+
+	it('LANG-U-EU-01: bare "eu" maps to eu', () => {
+		expect(detectLanguage('eu')).toBe('eu');
+	});
+
+	it('LANG-U-EU-02: regional variant "eu-ES" maps to eu', () => {
+		expect(detectLanguage('eu-ES')).toBe('eu');
+	});
+
+	it('LANG-U-EU-03: "eu-FR;q=0.5,en" — eu wins by primary preference', () => {
+		// q=0.5 is the lower bound we accept (mirrors AL-10 for es). The
+		// borderline value still beats `en` because the parser ranks by
+		// q-score and stops at the first known primary.
+		expect(detectLanguage('eu-FR;q=0.5,en;q=0.4')).toBe('eu');
+	});
+
+	it('LANG-U-EU-04: array of language tags — first eu wins over later en', () => {
+		// Mirrors AL-11. Validates the navigator.languages path for eu.
+		expect(detectLanguage(['eu-ES', 'en-GB'])).toBe('eu');
+	});
 });
diff --git a/apps/web/src/lib/utils/accept-language.ts b/apps/web/src/lib/utils/accept-language.ts
index 341e10c..c130bcb 100644
--- a/apps/web/src/lib/utils/accept-language.ts
+++ b/apps/web/src/lib/utils/accept-language.ts
@@ -1,21 +1,24 @@
 /**
  * Detect the user's preferred app language from an Accept-Language header
- * (or navigator.languages array) — Fase 10.8.
+ * (or navigator.languages array) — Fase 10.8 + Fase 20.3.2 (eu).
  *
- * Supported app languages (in sync with messages/{en,es}.json + Paraglide
- * languageTags): `en`, `es`. Anything else falls back to `en`.
+ * Supported app languages (in sync with messages/{en,es,eu}.json +
+ * Paraglide languageTags): `en`, `es`, `eu`. Anything else falls back
+ * to `en`.
  *
- * Rules (per plan §10.8.1):
+ * Rules (per plan §10.8.1, extended for Fase 20):
  *   * Inputs we accept:
- *     - bare tag: `'es'`, `'en'`, `'es-ES'`, `'en-US'`
- *     - RFC 7231 list: `'es;q=0.9,en;q=0.8'`
- *     - `navigator.languages` array: `['es-ES', 'en-GB']`
+ *     - bare tag: `'es'`, `'en'`, `'es-ES'`, `'en-US'`, `'eu'`, `'eu-ES'`
+ *     - RFC 7231 list: `'es;q=0.9,en;q=0.8'`, `'eu-FR;q=0.5,en;q=0.4'`
+ *     - `navigator.languages` array: `['es-ES', 'en-GB']`, `['eu-ES', 'en']`
  *     - null / undefined / empty → `'en'`
- *   * If the highest q-scored entry whose primary subtag is `es` has
- *     q ≥ 0.5, the result is `'es'`. Otherwise (including unknown
- *     languages, malformed input, or no clear preference) → `'en'`.
+ *   * The first entry (sorted by q-score descending) whose primary subtag
+ *     is an app-known language wins, provided q ≥ 0.5. `en` short-circuits
+ *     immediately regardless of q-score because it's the source language.
+ *   * Otherwise (including unknown languages, malformed input, or no
+ *     clear preference) → `'en'`.
  */
-export type AppLanguage = 'en' | 'es';
+export type AppLanguage = 'en' | 'es' | 'eu';
 
 interface ParsedTag {
 	tag: string;
@@ -76,11 +79,18 @@ export function detectLanguage(
 	// Sort by q-score descending.
 	parsed.sort((a, b) => b.q - a.q);
 
-	// First app-known primary wins; if it's es with q ≥ 0.5 → es.
+	// First app-known primary wins; non-`en` primaries require q ≥ 0.5 to
+	// beat the implicit `en` fallback. `en` short-circuits because it's the
+	// source language and can never be more wrong than the user's first
+	// choice if they listed it at all. Fase 20 adds `eu` on the same gate
+	// as `es`.
 	for (const p of parsed) {
 		if (p.primary === 'es') {
 			return p.q >= 0.5 ? 'es' : 'en';
 		}
+		if (p.primary === 'eu') {
+			return p.q >= 0.5 ? 'eu' : 'en';
+		}
 		if (p.primary === 'en') return 'en';
 	}
 
diff --git a/packages/test-utils/tests/language-bootstrap.test.ts b/packages/test-utils/tests/language-bootstrap.test.ts
index 278cc41..3dd339d 100644
--- a/packages/test-utils/tests/language-bootstrap.test.ts
+++ b/packages/test-utils/tests/language-bootstrap.test.ts
@@ -57,4 +57,58 @@ describe('Language bootstrap UPDATE', () => {
 		// Borja's seed language is 'es'; RLS must have blocked Ana's update.
 		expect(data?.language).toBe('es');
 	});
+
+	// ─── Fase 20.4.2 — Euskera bootstrap ─────────────────────────────────
+	// The bootstrap call sites build the UPDATE payload from the parser
+	// output of `detectLanguage(...)`. These exercise the live write of an
+	// `eu` value against the actual ENUM, and a negative case that the
+	// gate (the language_code enum from migration 028) rejects an unknown
+	// locale at write time so a careless client cannot persist a value the
+	// app has no messages for.
+
+	it('LANG-INT-EU-01: user can update their own language to eu', async () => {
+		const ana = await createClientAs(ANA_ID);
+		// Reset to en first via admin so the assertion measures the user
+		// write, not seed state.
+		await admin.from('users').update({ language: 'en' }).eq('id', ANA_ID);
+
+		const { error } = await ana
+			.from('users')
+			.update({ language: 'eu' })
+			.eq('id', ANA_ID);
+		expect(error).toBeNull();
+
+		const { data } = await admin
+			.from('users')
+			.select('language')
+			.eq('id', ANA_ID)
+			.single();
+		expect(data?.language).toBe('eu');
+	});
+
+	it('LANG-INT-EU-02: user trying language=\'fr\' is rejected by the enum', async () => {
+		const ana = await createClientAs(ANA_ID);
+		// Pre-set Ana to a known good value so a regression that silently
+		// accepts the bad UPDATE can't hide behind seed state.
+		await admin.from('users').update({ language: 'es' }).eq('id', ANA_ID);
+
+		const { error } = await ana
+			.from('users')
+			// Force any client-side narrow types out of the way — the gate
+			// lives in Postgres, not TypeScript.
+			.update({ language: 'fr' as unknown as 'en' })
+			.eq('id', ANA_ID);
+		// Postgres invalid_text_representation (22P02) for an unknown enum
+		// label; PostgREST surfaces it as a structured error.
+		expect(error).not.toBeNull();
+		expect(error?.code).toBe('22P02');
+
+		const { data } = await admin
+			.from('users')
+			.select('language')
+			.eq('id', ANA_ID)
+			.single();
+		// Untouched.
+		expect(data?.language).toBe('es');
+	});
 });