It’s just my thought.

For that case, you set the routes without language, and change middleware not to redirect leaving setting language as it is.

Honestly, I haven’t fully tested this thought.

/*
 * 1. sets i18n.locale and state.locale if possible
 * 2. redirects if not with locale
 */
export default function ({ 
  isHMR, app, store, route, params, error, redirect 
}) {
  // if (isHMR) { // ignore if called from hot module replacement
  //  return;
  // } // if url does not have language, redirect to english
  // else if (!params.lang) { 
  //  return redirect('/en'+route.fullPath);
  // }
  // based on directory structure _lang/xxxx, 
  // en/about has params.lang as "en"
  // const locale = params.lang || 'en'; 
  const locale = XXXXXXXXXXXXXXXXX; // read from cookie
  store.commit('SET_LANG', locale); // set store
  app.i18n.locale = store.state.locale;
}