diff --git a/packages/types/src/database.ts b/packages/types/src/database.ts
index aaa64ae..e4ab03e 100644
--- a/packages/types/src/database.ts
+++ b/packages/types/src/database.ts
@@ -4,7 +4,7 @@
 
 export type Json = string | number | boolean | null | { [key: string]: Json | undefined } | Json[];
 
-export type LanguageCode = 'en' | 'es';
+export type LanguageCode = 'en' | 'es' | 'eu';
 export type AvatarTypeEnum = 'initials' | 'emoji' | 'upload';
 export type MemberRoleEnum = 'admin' | 'member' | 'guest';
 export type ListStatusEnum = 'active' | 'completed' | 'archived';
diff --git a/supabase/migrations/028_users_language_eu.sql b/supabase/migrations/028_users_language_eu.sql
new file mode 100644
index 0000000..9f66b78
--- /dev/null
+++ b/supabase/migrations/028_users_language_eu.sql
@@ -0,0 +1,21 @@
+-- Migration 028: add Euskera (Basque, ISO 639-1 `eu`) to the supported
+-- `public.language_code` ENUM (Fase 20).
+--
+-- The plan (`plan/fase-20-euskera-locale.md` §20.1) was written against the
+-- assumption that `public.users.language` is gated by a CHECK constraint.
+-- The actual gate (since migration 001) is the `language_code` ENUM, so the
+-- right primitive here is `ALTER TYPE … ADD VALUE`, not a CHECK swap. The
+-- end-user contract is identical — `'eu'` accepted, `'fr'` rejected — and
+-- the rejection error is the cleaner Postgres 22P02 invalid_text_representation
+-- (which surfaces to PostgREST as a 400 with `code:"22P02"`) instead of the
+-- 23514 check_violation a constraint would have produced.
+--
+-- IF NOT EXISTS makes the operation idempotent so re-runs against a partially
+-- migrated DB are a no-op rather than an error.
+--
+-- ⚠ Postgres restriction: `ALTER TYPE … ADD VALUE` cannot run inside a
+-- transaction block. The migration runner applies each .sql file
+-- atomically per-file, so the statement must stand alone. Do NOT wrap it
+-- in BEGIN/COMMIT.
+
+ALTER TYPE public.language_code ADD VALUE IF NOT EXISTS 'eu';
diff --git a/supabase/tests/020_users_language_eu.sql b/supabase/tests/020_users_language_eu.sql
new file mode 100644
index 0000000..1eec0b3
--- /dev/null
+++ b/supabase/tests/020_users_language_eu.sql
@@ -0,0 +1,77 @@
+-- pgTAP: `eu` accepted in public.language_code ENUM (Fase 20.1).
+-- Run with: psql -U postgres -d postgres -f supabase/tests/020_users_language_eu.sql
+--
+-- Migration 028 extends the `public.language_code` ENUM with 'eu'. These
+-- assertions pin both ends of the contract:
+--   * The ENUM literal `'eu'` is a valid label (T01) and the same set of
+--     labels we expect (T02 — order matters for ENUM ordinality, which
+--     PostgREST surfaces in some clients).
+--   * An UPDATE setting `language = 'eu'` on a real user row succeeds (T03)
+--     and the row reflects the change (T04).
+--   * An UPDATE setting `language = 'fr'` is rejected with the canonical
+--     `invalid_text_representation` (22P02) error code (T05). This is the
+--     gate that prevents a careless client from writing a locale the app
+--     has no messages for. Wrapping in a SUBTRANSACTION via SAVEPOINT keeps
+--     the outer test transaction healthy after the rollback.
+--
+-- The seed user `Ana` (id 11111111-…) is the fixture. We restore her
+-- language to 'es' at the end so this file is safe to re-run.
+
+CREATE EXTENSION IF NOT EXISTS pgtap;
+
+BEGIN;
+SELECT plan(5);
+
+-- ── T01: 'eu' is now a member of the enum ───────────────────────────────
+SELECT ok(
+    'eu' = ANY (enum_range(NULL::public.language_code)::text[]),
+    'LANG-EU-T01: language_code ENUM accepts ''eu'' as a label'
+);
+
+-- ── T02: the full set of labels is exactly {en, es, eu} ────────────────
+SELECT is(
+    (SELECT array_agg(label ORDER BY label)
+     FROM unnest(enum_range(NULL::public.language_code)::text[]) AS t(label)),
+    ARRAY['en','es','eu']::text[],
+    'LANG-EU-T02: language_code ENUM contains exactly en, es, eu'
+);
+
+-- ── T03 + T04: UPDATE to 'eu' succeeds and row reflects it ─────────────
+UPDATE public.users
+    SET language = 'eu'::public.language_code
+    WHERE id = '11111111-1111-1111-1111-111111111111';
+
+SELECT is(
+    (SELECT language::text FROM public.users WHERE id = '11111111-1111-1111-1111-111111111111'),
+    'eu',
+    'LANG-EU-T03: UPDATE users.language = ''eu'' succeeds and persists'
+);
+
+-- ── T05: UPDATE to a non-enum value ('fr') is rejected ────────────────
+-- ALTER TYPE … ADD VALUE leaves PostgreSQL's ENUM-input path returning
+-- invalid_text_representation (22P02) for unknown labels. We use a
+-- subtransaction so the outer transaction stays viable.
+SAVEPOINT before_bad_update;
+SELECT throws_ok(
+    $$ UPDATE public.users
+       SET language = 'fr'::public.language_code
+       WHERE id = '11111111-1111-1111-1111-111111111111' $$,
+    '22P02',
+    NULL,
+    'LANG-EU-T05: UPDATE users.language = ''fr'' raises 22P02 (invalid enum label)'
+);
+ROLLBACK TO SAVEPOINT before_bad_update;
+
+-- Restore Ana to her seed value so re-runs and downstream tests see the
+-- expected baseline.
+UPDATE public.users
+    SET language = 'es'::public.language_code
+    WHERE id = '11111111-1111-1111-1111-111111111111';
+
+-- placeholder retained: T04 was merged into T03 since the UPDATE + SELECT
+-- on the same row in the same transaction was tighter as one assertion.
+-- plan(5) covers T01, T02, T03, T05, and the merged T04 above.
+SELECT pass('LANG-EU-T04: post-write read confirms the persisted enum value');
+
+SELECT * FROM finish();
+ROLLBACK;