Registration - include language in activation url

How can I add the current language to the activation url in the registration email?

For example, if a user selects a different language and then registers, but clicks the link in the confirmation email an hour later (so his session ended in the meantime), the confirmation page comes up in the default language, and not the one selected by the user.

You can change the line:
$app->redirect(hikashop::completeLink('checkout&task=activate_page',false,true));

by:
$lang = &JFactory::getLanguage();
$locale=strtolower(substr($lang->get('tag'),0,2));
$app->redirect(hikashop::completeLink('checkout&task=activate_page&lang='.$locale,false,true));

in the file administrator/components/com_hikashop/classes/user.php