l10nres.js

(function() {

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

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

  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') {

      console.log('element ready set');
      element.ready = document.l10n.languages.then(langs => {
        let currentLocale = [...langs][0];
        let href = element.getAttribute('href');
        href = href.replace('{locale}', currentLocale);
        return fetch(href);
      });

    }
  }
})();

l20n.js

let promises = [];


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

for (link of l10nResLinks) {
  console.log(link.ready);
  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);
});

langneg.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 languagesResolve;

  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 === 'META') {
      let name = element.getAttribute('name');
      if (name === 'defaultLanguage' || name === 'availableLanguages') {
        l10nMeta.set(name, element.getAttribute('content'));
        if (l10nMeta.size === 2) {
          observer.disconnect();
          negotiateLocales();
        }
      }
    }
  }

  function negotiateLocales() {
    localeChain.add('en-US');
    localeChain.add('fr');
    languagesResolve(localeChain);
  }

  document.l10n = {
    languages: new Promise(function(resolve, reject) {
      if (localeChain.size) {
        resolve(localeChain);
      } else {
        languagesResolve = resolve;
      }
    })
  };
})();

One general comment would be to figure out when we expect document.l10n.languages to be available. Maybe the shims need to be required in a specific order and the first one creates the document.l10n collection for the other shims to use?

Also a few coding comments which you may find useful:

let currentLocale = [...langs][0]; can also be written as let currentLocale = langs[0];

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

... can be replaced by:

const promises = Array.prototype.map.call(l10nResLinks, link => link.ready);

Also, you probably don't need to iterate twice with Promise.all here:

Promise.all(promises).then(function (resources) {
  return Promise.all(resources.map(function(response) {

Since response.text() and response.json() are also promises, you can achieve the same thing with a single Promise.all:

Promise.all(
  Array.prototype.map.call(l10nResLinks,
    link => doSomethingWithResponse(link.ready)))).then(..filter..);