function isLanguageSupported(lang) {
  var langSupported = false;
  if (lang == undefined) return false;
  jQuery.each(getAvailableLang(), function(index, val) {
    if (lang == val) {
      langSupported = true;
      return false; // leave the each-loop
    }
  });
  return langSupported;
}
This function checks if a given language is supported.
Useful with e. g. URL-Parameter
@param lang {String} Language like "de" or "en"
@return {Boolean} True / False