fallbackStack = function (langKey) {
    if (langKey) {
      if (angular.isString(langKey)) {
        $fallbackWasString = true;
        $fallbackLanguage = [ langKey ];
      } else if (angular.isArray(langKey)) {
        $fallbackWasString = false;
        $fallbackLanguage = langKey;
      }
      if (angular.isString($preferredLanguage)  && indexOf($fallbackLanguage, $preferredLanguage) < 0) {
        $fallbackLanguage.push($preferredLanguage);
      }

      return this;
    } else {
      if ($fallbackWasString) {
        return $fallbackLanguage[0];
      } else {
        return $fallbackLanguage;
      }
    }
  }
@ngdoc function
@name pascalprecht.translate.$translateProvider#fallbackLanguage
@methodOf pascalprecht.translate.$translateProvider

@description
Tells the module which of the registered translation tables to use when missing translations
at initial startup by passing a language key. Similar to `$translateProvider#use`
only that it says which language to **fallback**.

@param {string||array} langKey A language key.