No language override in email

Language overrides cannot work in emails on joomla 1.6.0 and higher. That's because of the restrictions put in place in the Joomla library and there is nothing we can do about that.
If you just use one language, you should make your modifications directly in the email via the menu System->Emails.

No, it should work with 1.5

It's in the function loadOrderNotification of the file administrator/components/com_hikashop/classes/order.php where the email is loaded.

Normally, the override is done automatically in joomla 1.6 and manually on 1.5 in the file amdinistrator/components/com_hikashop/helpers/helper.php :
$lang =& JFactory::getLanguage();
$override_path = JLanguage::getLanguagePath(JPATH_ROOT).DS.'overrides'.DS.$lang->getTag().'.override.ini';
if(version_compare(JVERSION,'1.6','=')&&$app->isAdmin() && file_exists($override_path)){
$lang->override = $lang->parse($override_path);
}
$lang->load(HIKASHOP_COMPONENT,JPATH_SITE);
if(version_compare(JVERSION,'1.6','<') && file_exists($override_path)){
$lang->_load($override_path,'override');
}

However, on joomla 1.6.1 and higher, the override system cannot be called from the back end. So all the backend and the emails sent from the back end cannot use the overridden translations.