I flew down to St Omer for the Jodel fly in (but we were in a C172) this year, and thankfully, through a modicum of French at school and a cheat sheet, we had no problem making the calls or understanding the basics of others in the circuit along with a very very very good lookout. Although that was pre-arranged, and there were a large number of UK flyers coming in for the event.
However, on the way back across the channel there were a couple of French pilots whose RT was absolutely disgraceful to the point the very busy ATC chap told one of them to just shut up until he called them again - didn't work and he stepped over another 4 people in succession (after doing it 3 times already).
Goes both ways...