code: function getLanguageFilter(languageId) {
          return function languageFilter(file) {
              var id = LanguageManager.getLanguageForPath(file.fullPath).getId();
              if (typeof languageId === "string") {
                  return (id === languageId);
              } else {
                  return (languageId.indexOf(id) !== -1);
              }
          };
      }
docs: Returns a filter for use with getAllFiles() that filters files based on LanguageManager language id
  @param {!(string|Array.<string>)} languageId a single string of a language id or an array of language ids
  @return {!function(File):boolean}