Locale resolution and URLs

On any server that owns its own request handler — a Vite-based framework, a Worker, a plain fetch handler — bind the locale per request. Next.js does this for you, through the proxy.

import { withRequestLocale } from 'best-i18n/server'

export default {
  async fetch(request: Request) {
    return withRequestLocale(request, I18N, () => handler.fetch(request))
  },
}

Resolution order: URL prefix → cookie → Accept-Language → base locale. The client mirrors the same order (resolveClientLocale) so hydration matches SSR. localizeUrl/deLocalizeUrl plug into router URL rewriting, with an exclude pattern for paths that must never be localized (/api/...).

Why there is no global locale on the server

setLocale from best-i18n/runtime is client-only, deliberately: one shared locale on a server would leak between concurrent requests. The server-side locale lives in AsyncLocalStorage, scoped to withRequestLocale / withLocale — and if the runtime cannot provide that isolation, the import throws instead of silently sharing state. On Cloudflare Workers that means enabling the nodejs_compat (or nodejs_als) compatibility flag; see Limitations.