l20n.js

let promises = [];

let l10nResLinks = document.head.querySelectorAll('link[rel=localization]');

for (link of l10nResLinks) {
  promises.push(link.ready);
};

Promise.all(promises).then(function (resources) {
  return Promise.all(resources.map(function(response) {
    if (!response.ok) {
      console.log('error fetching ' + response.url);
      return;
    }
    if (response.url.endsWith('.json')) {
      return response.json();
    } else {
      return response.text();
    }
  }).filter(n => n !== undefined));
}).then(function(responses) {
  console.log(responses);
});

webl10n.js

(function() {

  const observerConfig = {
    attributes: false,
    characterData: false,
    childList: true,
    subtree: false,
  };

  var observer = new MutationObserver(onMutations);
  observer.observe(document.head, observerConfig);

  let l10nMeta = new Map();
  let localeChain = new Set();
  let currentLocale;
  document.head.l10nResourceLinks = new Set();

  function onMutations(mutations) {
    for (let mutation of mutations) {
      for (let addedNode of mutation.addedNodes) {
        if (addedNode.nodeType === Node.ELEMENT_NODE) {
          onAddedHeadElement(addedNode);
        }
      }
    }
  }

  function onAddedHeadElement(element) {
    if (element.nodeName === 'LINK' &&
        element.getAttribute('rel') === 'localization') {
      document.head.l10nResourceLinks.add(element);
      if (currentLocale) {
        initLinkLoading(element);
      }
    } else if (element.nodeName === 'META') {
      let name = element.getAttribute('name');
      if (name === 'defaultLanguage' || name === 'availableLanguages') {
        l10nMeta.set(name, element.getAttribute('content'));
        if (l10nMeta.size === 2) {
          startL10n();
        }
      }
    }
  }

  function negotiateLocales() {
    localeChain.add('en-US');
    localeChain.add('fr');
    return Promise.resolve();
  }

  function initResourceLoading() {
    currentLocale = [...localeChain][0];
    for (link of document.head.l10nResourceLinks) {
      initLinkLoading(link);
    }
  }

  function initLinkLoading(link) {
    let href = link.getAttribute('href');
    href = href.replace('{locale}', currentLocale);
    link.ready = fetch(href).then(function(response) {
      return response; 
    });
  }

  function startL10n() {
    negotiateLocales().then(initResourceLoading);
  }
})();