Seen your questions, I might suggest that you build a little flying experience before going to fly outside your traffic pattern ?
Apart from flight plans, should I be wary of non-english speaking controllers
All french Controllers speak English when adressed in English.. If you stick to Controlled airfileds and Flight info frequencies , you will have no problems. If you venture into Airfilelds with A/A outside ATS hours and FR only airfileds ( look at the VACs ) there you might/will get into trouble.
Also, is there any truth in some rumours that in early July there are a lot of thunder storms in France with embeded CBs
Yes, the same rumors goes as saying that when you have low very dark clouds in late November , it normally rains as well.