The reason why the French won't speak in anything other than French is a cultual arrogance that seems to be taught from birth and despite the writing (six foot high in English!) on the wall they just cant get over the attitude that France is exclusively for the French and the rest of Europe should be the same way.
Eaxample. Last year I was employed by a UK airline on contract to a German airline working a French airline contract based in Lille.
When this became common knowlage a local newspaper started an investigation, the main thrust of this was French job protection. I had a brief chat with the investigating jorno and his attitude was that "something had to be wrong or illegal because the pilots were not French".
I pointed out to him (with a wry smile) that we all now lived in the United States of Europe and that this was governed from Brussels, the result was that we could all life and work in any part of Europe. This came as somewhat of a shock when he realsed that the EEC was "bigger" than France, not run by France and that the French had let the EEC cat out of the bag and worse could not put the cat back!
It was great fun flying the skys of Europe with a French cabin crew in the French all electric jet with a "Starway" c/s.
As "Starway" is a French airline French was the usual responce from ATC and I was getting quite good at understanding ATC commands in French, this only Re-enforced my opinion that the use of more than one language for ATC is a flight safety disaster in the making.....................Hell if the biggest nation on the planet (China) will comply with the ICAO standard languge why cant a the French.............. now thats what I call arrogance !