Hi,
There is no such "ORDER_EMAIL_PAYREMINDER" translation key in the language file of HikaShop.
The translation key is INCLUDE_ORDER_NUMBER_TO_TRANSFER and you can find it in the language file of HikaShop:
INCLUDE_ORDER_NUMBER_TO_TRANSFER="Do not forget to include the order number ( %s ) in the comments of the bank transfer so that we can match it to your order."
And in the preload of the order creation notification email you can also see this translation key being used:
echo '<p>' . JText::sprintf('PLEASE_TRANSFERT_MONEY',$amount).'<br/>'.$information.'<br/>'.JText::sprintf('INCLUDE_ORDER_NUMBER_TO_TRANSFER',$data->order_number) . '</p>';
So I'm not sure how you were able to have what you're describing.
I suppose that you have some old override of the preload of that email which uses a translation we don't use anymore.
In the override of the preload, you must have something like that: JText::_('ORDER_EMAIL_PAYREMINDER'.... while it should be:
JText::sprintf('INCLUDE_ORDER_NUMBER_TO_TRANSFER'...