Vite

The Vite plugin covers Vite itself and anything running on it: TanStack Start, React Router, SvelteKit, Astro. rolldown-vite keeps the Vite plugin API, so it uses this plugin unchanged.

// vite.config.ts
import process from 'node:process'
import { fileURLToPath } from 'node:url'
import { i18n } from 'best-i18n/vite'

export default defineConfig({
  plugins: [
    i18n({
      messagesDir: fileURLToPath(new URL('./messages', import.meta.url)),
      locales: ['en', 'zh'],
      baseLocale: 'en',
      // set (e.g. from an env var) to build a single locale as pure literals
      staticLocale: process.env.I18N_STATIC_LOCALE,
    }),
    // ...other plugins
  ],
})

The .po files under messagesDir are the source of truth — the plugin reads them directly, no compile step in between. Editing a message file in dev triggers a full reload.

With staticLocale set, the build inlines that one locale as plain string literals — one deployable bundle per locale, each with zero i18n overhead. See Writing messages for what compiles, and Extract and translate for the catalog workflow.