code: function translateFragment(element) {
      element = element || document.documentElement;
  
      // check all translatable children (= w/ a `data-l10n-id' attribute)
      var children = getTranslatableChildren(element);
      var elementCount = children.length;
      for (var i = 0; i < elementCount; i++) {
        translateElement(children[i]);
      }
  
      // translate element itself if necessary
      translateElement(element);
    }
docs: translate an HTML subtree