Contao\System::isInstalledLanguage PHP Method

    public static function isInstalledLanguage($strLanguage)
    {
        if (!isset(static::$arrLanguages[$strLanguage])) {
            if (is_dir(TL_ROOT . '/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];
    }