I believe the person who tried to impose this rule is no longer in his previous position of authority, although regretably for those who have to share the crew room / cockpit with him , neither is he in the Caribean as planned. . . . shame that.
If I had found myself in receipt of his "advice" I would have spoken French to whoever was around, whether they understood or not, even if English ( or at least my dialect of it) is my mother tongue.
I suggest you use the time spent in his presence to insult him among yourselves in a language he doesn't understand, it can be most rewarding.