[mforce]

Remember the last UI language on this device. Fixes two related reports: (1) the login screen always rendered English even after choosing Spanish/Tagalog, and (2) a signed-in Spanish user who pressed F5/Ctrl+R momentarily reverted to English (only a cache-busting Ctrl+Shift+R "stuck" to Spanish).

Root cause

The SPA had no client-side language memory. Every load booted i18next in English and only switched after /me resolved:

• Login screen is outside ProtectedRoute — no /me to resolve from — so it was unavoidably English (the i18n infrastructure (SPA foundation): react-i18next + resolution + string sweep (tracker) #182 plan explicitly left room for "a persisted non-sensitive local language hint").
• Signed-in refresh started English, then the bootstrap switched to the /me language. On a warm F5 that English starting point paints first and lingers; a cold hard-reload hides it behind the loading state — exactly the F5-vs-hard-reload split. (/me is not service-worker cached — runtimeCaching: [], and the navigate-fallback denylist excludes /api — so this was never stale data, only the English starting point.)

Fix

• web/src/lib/languageHint.ts — a device-local hint over localStorage["cluckwork.lang"] (readLanguageHint/writeLanguageHint, both storage-guarded) + a pure pickInitialLanguage(hint, supported, fallback) that validates the hint against the installed packs (case-insensitive, mirroring resolve.ts; garbage/removed code → English).
• web/src/i18n/index.ts — seed i18next's initial lng from the hint, and persist the hint on every languageChanged. i18next.init is synchronous with in-memory resources, so first paint already renders in the remembered language — no flash, no pre-paint script. The languageChanged listener is registered before init() so it catches init's own synchronous first event and sets <html lang> correctly on a hinted boot.

/me stays the source of truth: after sign-in, SessionProvider still resolves from /me + /account and overrides the hint (behind the existing render gate — no flash), and the listener writes that resolved value straight back, so the hint self-heals to the authoritative preference. Device-persistent like cluckwork.theme (not cleared on logout — a returning user meets their own language). Non-sensitive: a language code only; no interaction with the memory-only token store (#145).

Tests

• web/src/lib/languageHint.test.ts — round-trip, storage-throw guards, and pickInitialLanguage (supported / null / garbage / case-insensitive).
• web/src/i18n/languageHint.integration.test.ts — the real singleton's languageChanged persists the hint and keeps <html lang> in sync.
• web/src/test/setup.ts — reset now runs before localStorage.clear() so the listener's hint write is wiped → every test starts hint-free.

npm run test 1129 green · npm run typecheck clean · npm run build succeeds.

Review

Reviewed (Claude); first pass caught a real defect — the languageChanged listener was registered after init(), so it missed init's synchronous first event and left <html lang> at en on a hinted boot; the seeding test's resetModules didn't actually isolate i18next and masked it. Both fixed (listener reordered; seeding extracted to a pure unit-tested function; integration test rewritten to be honest about what it exercises). Re-review: RESOLVED. One residual known-gap: no single end-to-end cold-boot test through index.ts (i18next is externalized, so Vitest can't cleanly reload it) — covered by the pure-logic unit test + the listener test + an out-of-band probe.

Not included

Separate from the #182 string sweep (its final PR is #252). Unrelated local working-tree edits (deploy/docker-compose.yml, web/vite.config.ts) are deliberately not in this branch.