Service Worker gotchas – https://medium.com/kollegorna/service-worker-gotchas-af20a9dab986

service-worker-serve-multilingual-offline-page.js

const appLangs = ['en', 'sv'],
      url      = new URL(event.request.url);

event.respondWith(
  fetch(event.request).then(response => {
    // ...
    return response;
  })
  .catch(function() {
    let lang = url.pathname.substr(1, 2);
    lang = appLangs.includes(lang) ? lang : appLangs[0];
    return caches.match(`/${lang}/offline/`);
  })
);