Hi,
I think it's a problem in the code.
In the file administrator/components/com_hikashop/classes/order.php we have this code:
$order->order_url = hikashop_contentLink('order&task=show&cid[]='.$order->order_id.$url, $order, false, false, false, true);
$app = JFactory::getApplication();
$locale = $this->loadLocale($order);
The call to the loadLocale function is what properly sets the current language to the language of the customer. But it is placed after the line setting $order->order_url which contains the URL of the order.
So I think this code should be changed to swap both lines, like this:
$locale = $this->loadLocale($order);
$order->order_url = hikashop_contentLink('order&task=show&cid[]='.$order->order_id.$url, $order, false, false, false, true);
$app = JFactory::getApplication();
It should then generate a link with the correct language.