We actually have such code to get the front end user language.
But that only works when the user sets his language.
We also have some code to get the front end default language but it seems to not work anymore on new versions of Joomla.
Please change the code (there are 2 places where you need to do that):
$config =& JFactory::getConfig();
$locale = $config->getValue('config.language');
to:
$params = JComponentHelper::getParams('com_languages');
$locale = $params->get('site', 'en-GB');
in the file administrator/components/com_hikashop/classes/order.php
and that should load the correct front end language.