Backend\Core\Language\Language::setWorkingLanguage PHP Method

    public static function setWorkingLanguage($language)
    {
        self::$currentWorkingLanguage = (string) $language;
    }

 /**
  * @param string $language
  *
  * @throws Exception If the provided language is not valid
  */
 public function setLanguage($language)
 {
     // get the possible languages
     $possibleLanguages = BackendLanguage::getWorkingLanguages();
     // validate
     if (!array_key_exists($language, $possibleLanguages)) {
         throw new Exception('Language invalid.');
     }
     // set working language
     BackendLanguage::setWorkingLanguage($language);
 }