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);
            }
        };
    }
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}