Hola S&N:
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Before writing the rest of this, I agree with you regarding the use of English, but try to convince the French to do so. My concern is same - that many crews do not understand French. I am myself Brussels-born, so I can bark in many languages and understand others... and sometimes in French airspace, I have (bad boy) used French with ATC to get some "favors" such as negotiating a "clear direct to" a waypoint.
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I used to be with PanAm when I lived in USA... flying frequently in French airspace. I had myself an incident during an approach to Nice in the 1970s, where an Air Inter Caravelle was cleared (in French) to descent and hold at the same point and altitude we were holding ourselves, and immediately advised ATC and the Air Inter in French that we were still at that altitude. Would I have not known French...
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Now flying in Argentina, I have to say that I often use Spanish over South America, and even Portuguese over Brasil. But I would never do so when I hear an American or European flight on the frequency... and at times, I translate ATC advisories (i.e. turbulence reports) into English for the benefit of other flights who probably would fail to understand.
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If you ever hear me speaking French with ATC in France, it is merely to ask the latest of championship soccer or rugby games... Like you said, most French pilots are able to converse in very decent English... but it is their airspace. Until a grave incident happens (I hope just an incident), they will probably continue to do so.
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Since you are Danish - I miss the great and inexpensive "biksemad" I had near the Kastrup terminal when I worked for Sterling... was delicious. Mange tak for the hospitality you guys gave us there, and in Billund as well.
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Happy contrails