Based on the tragic crash of a light aircraft crossing the Channel on another thread: Do
not fly over, through, or under any CBs. Only ever fly round them in the horizontal plane - and keep well away from them as you do so. Remember CBs might be embedded in other cloud so you may not know it is there without a good weather radar, and experience in using it.
French ATC is generally very good, helpful and clear, but I would always speak only English, and use slow CAP 413 phraseology - don't engage in any French or chatting. You want the controller's brain to switch to English straight away. If you start with French phrases and your accent is good, (mine is apparently), ATC might reply in French and/or it might take a moment for them to realise and switch. But by all means finish your final message with. "Merci beacoup, á le prochain", or whatever.
Have a good flight, and above all do not be afraid or too proud to turn 180° and go home if you don't feel comfortable or anything is not right - don't press on regardless
