I stumbled on this thread while researching similar topics.
As an English teacher who works with pilots, I can say that, in my experience, French ATC have good English language skills (at least in my region!)
On "practical" language sessions, involving flying circuits at BDX, ATC has been more than willing to speak with my student in English, despite the strong gallic twang and F-registered aircraft!
On the other side of the coin, as I train for my PPL down here, all RT is done in French, regardless of whether or not we are using an "English" field. I feel that since I can speak French, I should, out of respect.