CRestUtil::getLanguage

static function getLanguage()
{
	$languageId = '';

	$siteIterator = BitrixMainSiteTable::getList(array(
		'select' => array('LANGUAGE_ID'),
		'filter' => array('=DEF' => 'Y', '=ACTIVE' => 'Y')
	));
	if($site = $siteIterator->fetch())
	{
		$languageId = (string)$site['LANGUAGE_ID'];
	}

	if($languageId == '')
	{
		if(BitrixMainLoader::includeModule('bitrix24'))
		{
			$languageId = CBitrix24::getLicensePrefix();
		}
		else
		{
			$languageId = LANGUAGE_ID;
		}
	}

	if($languageId == '')
	{
		$languageId = 'en';
	}

	return $languageId;
}