Contao\CoreBundle\Framework\ContaoFramework::setDefaultLanguage PHP Method

    private function setDefaultLanguage()
    {
        $language = 'en';
        if (null !== $this->request) {
            $language = str_replace('_', '-', $this->request->getLocale());
        }
        // Deprecated since Contao 4.0, to be removed in Contao 5.0
        $GLOBALS['TL_LANGUAGE'] = $language;
        $_SESSION['TL_LANGUAGE'] = $language;
    }