And now this makes me wonder about T-V distinction in Starfleet. I'm sure there are regulations for that and Picard being the captain, kind of gives him the "right" to tutoyer everyone on the ship while still needing to vouvoyer admirals.
Weirdly my native language is French but I have always only seen Star Trek in English so I don't know how they dub it. Somehow I would have thought that everyone would be formal, polite and use vous, but maybe the captain can say tu to other people. Or he's very friendly with Riker.
My theory is that France got it bad during WW3, and many of the survivors relocated to the UK until the fallout could be cleaned up, which is why Jean-Luc Picard sounds like that.
It's england's revenge for the norman conquest. The normans gave the english a whole bunch of new words and weird pronunciations. The brits just figured turnabout is fair play.