Nuxt/examples/i18n/middleware/i18n.js

export default function ({ isHMR, app, store, route, params, error, redirect }) {
  const defaultLocale = app.i18n.fallbackLocale;
  // If middleware is called from hot module replacement, ignore it
  if (isHMR) return
  // Get locale from params
  const locale = params.lang || defaultLocale
  if (store.state.locales.indexOf(locale) === -1) {
    return error({ message: 'This page could not be found.', statusCode: 404 })
  }
  // Set locale
  store.commit('SET_LANG', locale)
  app.i18n.locale = store.state.locale
  // If route is /<defaultLocale>/... -> redirect to /...
  if (locale === defaultLocale && route.fullPath.indexOf('/'+defaultLocale) === 0) {

    var toReplace = '^/'+defaultLocale
    var re = new RegExp(toReplace); 
    return redirect(
      route.fullPath.replace(re, '/')
    )
  }
}