Supposedly you can kill servants by recreating certain of conditions of how they originally died. That said, if you try having Artoria try stabbing Mordred with Rhongonmyiad, Mordred is just going to laugh at you.
Astolfo is English though. But then again, he might have picked up a few things from his Frankish friends =w=
He is a character originating from a fiction titled "Matter of France", and seems to follow French culture over his birth background. Or at least in the context of this story.