public static function getMessage( $code, $lang = 'EN' ) {
    if( $code < 0 ) {
      return null;
    } //code invalid

    $lang = strtoupper( $lang );

    if( !array_key_exists( $lang, self::$code ) ) { //language not found
      $lang = 'EN';
    }

    if( array_key_exists( $code, self::$code[$lang] ) ) { //code not defined
      return self::$code[$lang][$code];
    }

    return null;
  }