i18n

name	i18n
description	i18n patterns for an AdonisJS + Inertia app. Keys namespaced by module, `useTranslation()` on the frontend and `ctx.i18n.t(...)` on the backend, locale persisted per user with a cookie fallback. Every new key needs an entry in every supported locale JSON. Use when adding UI strings, wiring a new module's translations, or debugging locale detection. Trigger on: "add translation", "i18n", "locale", "translate", "add key".
license	MIT
allowed-tools	Read, Edit, Write, Bash, Grep, Glob

i18n

Locales are declared in config/i18n.ts (supportedLocales, defaultLocale, loaders). Translation JSON lives per module at app/<mod>/resources/lang/<locale>/<mod>.json. Keys are namespaced by module (common.layout.navMain.dashboard), so the file at app/common/resources/lang/en/common.json doesn't repeat common. inside — the loader adds it. Backend uses ctx.i18n.t('key'); frontend uses useTranslation(). Locale detection runs as an HTTP middleware that prefers an X-User-Language header → a user-locale cookie → the browser's Accept-Language. Authenticated users have User.locale as the authoritative source: the switch endpoint writes both cookie and DB; every login flow syncs the cookie from user.locale if set.

Rules

• Adding a key requires an entry in every supported locale. There is no auto-generate. Missing keys fall back to the default locale at runtime — the mixed-language UI is the symptom.
• Key naming: <mod>.<section>.<key>. The <mod> prefix comes from the file location; inside <mod>.json you don't repeat it. E.g. inside common.json:

  { "layout": { "navMain": { "dashboard": "Dashboard" } } }
  

  and consumed as t('common.layout.navMain.dashboard').
• Frontend: useTranslation() returns { t, changeLanguage, language }. t(key, params?) supports ICU interpolation.
• Backend: ctx.i18n.t('key', params?). The i18n instance is bound to HttpContext by the locale-detection middleware. Use in controllers, actions (if i18n was in the input), and mail classes.
• ICU interpolation: {name} in the string, t('key', { name: 'Alice' }) at the call site. Also supports plurals and selectors.
• Validation messages: VineJS uses the request's i18n instance through the messages provider bound in the locale-detection middleware. Localize by adding keys under <field>.<rule> in <mod>/resources/lang/<locale>/validator.json.
• User preference: User.locale is a nullable column. When authenticated, the switch endpoint updates both the cookie and user.locale. Login flows sync user-locale cookie from user.locale if set. Pre-login (marketing/auth pages) uses cookie / Accept-Language, never DB.
• Locale switcher UI: <LanguageSwitcher /> lives in every logged-in shell — see [[layout-shells]].
• Adding a locale means updating supportedLocales in config/i18n.ts, adding a folder under every module's resources/lang/, and confirming every key exists in the new locale.

Repo refs

• Locale config + per-module loaders: config/i18n.ts. Keys: app/users/resources/lang/{en,fr,pt}/users.json.
• Frontend hook: app/common/ui/hooks/use_translation.ts. Detection middleware: app/core/middleware/detect_user_locale_middleware.ts.

Doc refs

• AdonisJS i18n — https://docs.adonisjs.com/guides/digging-deeper/i18n
• ICU message format — https://docs.adonisjs.com/guides/digging-deeper/i18n#icu-message-format
• Translating validator errors — https://docs.adonisjs.com/guides/digging-deeper/i18n#translating-validation-messages

Workflow

Add a translation key

1. Pick the module. If the string is shared UI (nav, buttons, layout), put it in common.
2. Open the JSON file for every supported locale: app/<mod>/resources/lang/<locale>/<mod>.json.
3. Add the same key path in every file with the localized value.
4. Frontend: t('<mod>.<path>') via useTranslation().
5. Backend: ctx.i18n.t('<mod>.<path>').
6. Verify parity: grep the key across app/<mod>/resources/lang/*/ — it must appear in all three locales. Present in only one → it falls back silently and ships a mixed-language UI.

Add a new module's translations

1. Create app/<mod>/resources/lang/<locale>/<mod>.json for every supported locale.
2. Add an fs loader entry in config/i18n.ts:

   loaders.fs({ location: app.makePath('app/<mod>/resources/lang') }),
   

3. The frontend glob picks the files up automatically.

Interpolation

{ "welcome": "Welcome, {name}!" }

t('common.welcome', { name: user.fullName })

Plural / selector:

{ "items": "{count, plural, =0 {No items} one {# item} other {# items}}" }

Localize a validator message

Add to app/<mod>/resources/lang/<locale>/validator.json:

{ "required": "This field is required" }

VineJS looks these up automatically because the messages provider is bound in the locale-detection middleware.

Set a user's default locale programmatically

Use during invite / sign-up when the target's language is known:

await User.create({ email, locale: 'pt' })

The next login flow writes the cookie for them.

Anti-patterns

• ❌ Adding a key in one locale but not the others — runtime falls back silently to the default and users see mixed languages.
• ❌ Hardcoding strings in components (<span>Save</span> instead of t('common.save')).
• ❌ Reading req.cookies['user-locale'] directly instead of ctx.i18n.locale — bypasses the detection priority (header → cookie → Accept-Language).
• ❌ Persisting locale to the cookie without also updating user.locale when the user is authenticated — the switch endpoint handles both; don't reimplement.
• ❌ Deep key paths (common.a.b.c.d.e.f) — collapse the JSON tree, keep depth ≤ 3.
• ❌ Repeating the module prefix inside the JSON file (common.json with a common root) — the loader adds it.

Related skills

[[inertia]] · [[layout-shells]] · [[mail]] · [[crud]] · [[module-scaffolding]]