When I was at a restaurant in Mestre, Italy, there was a Russian couple a few tables over. They kept looking over at us and making rude comments about me (loudly, in Russian).
In Bulgarian, I asked if they would tone it down, people were trying to enjoy their dinner and smiled when I asked.
He replied with some expletives and said "we can go outside anytime".
I smiled and said (quietly, in unmistakable American-english), "you want to f*ck around, we'll do it here and after I beat your stupid ass, I'll make you pay everyone's bill. Now shut the f*ck up and eat while you still have teeth".
Aparently, he decided dinner was more important and let it go.
By the way, the food in Mestre is excellent.