Hi,
It indeed seems to be the case.
When I search for ORDER_NOTIFICATION_SUBJECT I can see this line for the Slovenian:
ORDER_NOTIFICATION_SUBJECT="Naročilo št. %s je %s - %s"
But if I check the same translation key for other languages, I see this:
ORDER_NOTIFICATION_SUBJECT="Your order No. %s at %s"
So there indeed is an extra parameter in Slovenian which shouldn't be there. Remove it and it should help.
We'll also change it on our end.
I think it was a bad copy/paste from one of the other order email subjects by the person who made that translation.