code: public static function isInstalledLanguage($strLanguage)
  	{
  		if (!isset(static::$arrLanguages[$strLanguage]))
  		{
  			$rootDir = self::getContainer()->getParameter('kernel.project_dir');
  
  			if (is_dir($rootDir . '/vendor/contao/core-bundle/src/Resources/contao/languages/' . $strLanguage))
  			{
  				static::$arrLanguages[$strLanguage] = true;
  			}
  			elseif (is_dir(static::getContainer()->getParameter('kernel.cache_dir') . '/contao/languages/' . $strLanguage))
  			{
  				static::$arrLanguages[$strLanguage] = true;
  			}
  			else
  			{
  				/** @var SplFileInfo[] $files */
  				$files = static::getContainer()->get('contao.resource_finder')->findIn('languages')->depth(0)->directories()->name($strLanguage);
  				static::$arrLanguages[$strLanguage] = \count($files) > 0;
  			}
  		}
  
  		return static::$arrLanguages[$strLanguage];
  	}
docs: Check whether a language is installed
  
  @param boolean $strLanguage The language code
  
  @return boolean True if the language is installed