you're right, they prefer to play with their friends. It's like the japanese who kicked every non-japanese players... I guess they like their french community, they even have their own mini-games inside the game and all... French players begin to be very numerous. But I agree, this is somewhat childish (I think most of them are indeed very young) !
Anyway, it happens I play with them (I'm french by the way ;-) but I'm 26 and like to play with american/english as well !) and it's definitely better when you can chat and comment the games. I'll ask them why they do that systematically.