But a French controller at CDG wouldn't try talking to a UK reg in French nor to a D reg in German, English would be the best option, even though ICAO Annex 10 Vol 11 para 5.2.1.2 stipulates French, with English on request.
I´ve been adressed in french more than once around Paris in D-Reg airplanes. When things get hectic - as they sometimes do in this very busy airspace - the switching between languages puts more strain on the controllers than necessary.
I would guess that most non-native english speakers (myself included) find english very easy to use especially when used properly in aviation. And it is definetaley easier to learn than french - I´ve tried to learn both.