Languages and flight safety don't go together. I understand the point you are making about an english speaking pilot speaking English at a French speaking airfield, but what if there was an urgent reason to land? Maybe this is a question for seasoned european tourers; if you plan a flight from x to y with alternate z (all ATS airports), and then one of your passengers became seriously ill, would you continue to the destination or alternate if the nearest airfield was a French speaking one?
I know legally you are not supposed to land at French airfields if you are not proficent in French, but would something like this be an exception? In the OPs example of his student flying solo, he'd probably have no clue what you were doing.
I have seen other threads about English speaking pilots flying into ATS airports in France, where French traffic spoke only French. Not good for avoiding conflict.
Wouldn't it be easier if we all just spoke English!
