zebo - Always wonderful if you can embarrass people who are being rude to you because they think you don't understand.
I recall queuing in a local bookshop to be served, and in front of me was a group of German teenagers, possibly on an exchange. The gobbiest one said something to his friends in German in a loud voice, and they all laughed. The assistant then said something in German to him, and he went crimson and quiet as a mouse.
When I got to the till, I asked her what the exchange was.
The assistant told me that the student had said in German, "If the fat cow behind the counter gets a move on, maybe we can get out of here.
And she said, also in in German, "The fat cow is working as fast as she can, and you will be served as soon as possible."
Priceless.
I also remember the gorgeous teenage assistant at our local café advising me that when she went on holiday to Spain, she enjoyed putting the young locals in their place in Spanish when they talked about her - her dad was Spanish and she spoke it fluently, including the appropriate salty language for the occasion!