code: public static function getTinyMceLanguage()
  	{
  		$lang = $GLOBALS['TL_LANGUAGE'];
  
  		if ($lang == '')
  		{
  			return 'en';
  		}
  
  		$lang = str_replace('-', '_', $lang);
  		$rootDir = System::getContainer()->getParameter('kernel.project_dir');
  
  		// The translation exists
  		if (file_exists($rootDir . '/assets/tinymce4/js/langs/' . $lang . '.js'))
  		{
  			return $lang;
  		}
  
  		if (($short = substr($GLOBALS['TL_LANGUAGE'], 0, 2)) != $lang)
  		{
  			// Try the short tag, e.g. "de" instead of "de_CH"
  			if (file_exists($rootDir . '/assets/tinymce4/js/langs/' . $short . '.js'))
  			{
  				return $short;
  			}
  		}
  		elseif (($long = $short . '_' . strtoupper($short)) != $lang)
  		{
  			// Try the long tag, e.g. "fr_FR" instead of "fr" (see #6952)
  			if (file_exists($rootDir . '/assets/tinymce4/js/langs/' . $long . '.js'))
  			{
  				return $long;
  			}
  		}
  
  		// Fallback to English
  		return 'en';
  	}
docs: Return the TinyMCE language
  
  @return string