Nuxt/examples/i18n/middleware/i18n.js

export default async function ({ store, params, error }) {
  const lang = params.lang || 'en'
  if (!store.state.lang.locales.includes(lang)) {
    await store.dispatch('lang/setLang', 'en')
    return error({ message: 'Page not found', statusCode: 404 })
  }
  await store.dispatch('lang/setLang', lang)
}