Internationalization Best Practices

Guidelines for building a React Router i18n setup with remix-i18next. Focuses on middleware detection, locale storage, type safety, and client/server synchronization.

When to Apply

Adding i18n to a React Router app
Wiring remix-i18next middleware
Implementing language switching or locale detection
Serving locale resources from /api/locales

Rules Summary

Setup & Middleware (CRITICAL)

setup-middleware - @rules/setup-middleware.md

Configure createI18nextMiddleware and type-safe resources.

export const [i18nextMiddleware, getLocale, getInstance] =
  createI18nextMiddleware({
    detection: {
      supportedLanguages: ["es", "en"],
      fallbackLanguage: "en",
      cookie: localeCookie,
    },
    i18next: { resources },
    plugins: [initReactI18next],
  });

locales-structure - @rules/locales-structure.md

Define locale resources per language and re-export.

// app/locales/en/translation.ts
export default { title: "Example" };

Namespaces (HIGH)

namespaces-strategy - @rules/namespaces-strategy.md

Use a single namespace for small apps; multiple namespaces for large apps.

// Large app: common + route namespaces
export default { common, home, notFound };

Locale Detection & Persistence (CRITICAL)

locale-detection - @rules/locale-detection.md

Prefer cookie/session for speed, with DB as source of truth.

export const [i18nextMiddleware, getLocale] = createI18nextMiddleware({
  detection: { cookie: localeCookie, fallbackLanguage: "en" },
});

language-switcher - @rules/language-switcher.md

Store locale in cookie/session and keep it in sync.

return data(
  { locale },
  { headers: { "Set-Cookie": await localeCookie.serialize(locale) } },
);

Client & Server Integration (CRITICAL)

root-locale-sync - @rules/root-locale-sync.md

Send locale to the UI and sync <html lang dir>.

export async function loader({ context }: Route.LoaderArgs) {
  let locale = getLocale(context);
  return data(
    { locale },
    { headers: { "Set-Cookie": await localeCookie.serialize(locale) } },
  );
}

entry-client-init - @rules/entry-client-init.md

Initialize i18next client with htmlTag detection.

i18next.init({ detection: { order: ["htmlTag"], caches: [] } });

entry-server-provider - @rules/entry-server-provider.md

Reuse the middleware instance in SSR with I18nextProvider.

<I18nextProvider i18n={getInstance(routerContext)}>
  <ServerRouter context={entryContext} url={request.url} />
</I18nextProvider>

Resource Routes & Caching (HIGH)

locales-resource-route - @rules/locales-resource-route.md

Serve /api/locales/:lng/:ns with validation and cache headers.

return data(namespaces[ns.data], { headers });

UI Usage (MEDIUM)

use-bound-t-in-loader - @rules/use-bound-t-in-loader.md

Use the bound t() in loaders and useTranslation in components.

let t = getInstance(context).getFixedT(locale);

Not Found (MEDIUM)

not-found-i18n - @rules/not-found-i18n.md

Provide a 404 route so middleware runs and translations load.

/frontend-internationalization-best-practices

SKILL.md