setupPreferredLanguage = function (langKey) {
    if (langKey) {
      $preferredLanguage = langKey;
    }
    return $preferredLanguage;
  }
@ngdoc function
@name pascalprecht.translate.$translateProvider#preferredLanguage
@methodOf pascalprecht.translate.$translateProvider

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

@param {string} langKey A language key.