Wait. What if it happened int he mansion too. That its too early for anyone to properly process French or any other language for the matter.
Napoleon, Jean and Comte being the only ones who can properly process it 24/7
Will has been in France long enough to be okay with the language, but since he lives alone and talks to himself in English, he'll go out and start speaking English and then remember where he is.
Luckily the brother have each other, and Dazai and Sebastian have each other.
For Leonardo you just need to speak a little slower.
Leonardo: Comte, aspetta. Un po' più lento, per favore(Comte, wait. Little slower, please)
Issac grading papers can be a disaster because he starts writing in English and he's like OH SHIT, but he does it in class. He'll forget the french word and writes it in English and hopes no one notices.
But even Jean and Napoleon speak different dialects, so sometimes its hard for them.
When Vincent wakes up int he middle of the night and see's Napoleon, and Napoleon just starts having a conversation. First off, Vincent is half asleep, he can't understand a single word, but smiles and nobs and just replies back in dutch saying "Thank you. Goodnight" and turns around and goes back to bed.
In the music room you just hear Mozart yelling in German for a different number of reasons. Doesn't matter the time of day. Mozart's brain doesn't flick to French mode until someone starts talking to him in French. Then he goes "Wait. I'm in France. Ew"
Dazai gives me vibes that he never even bothered to learn French. Sebastian knows it, but some days he's not feeling it and takes him like 5 minutes to process what you said, and to think of something to say.
Faust and Mozart meet and they are so happy.
Faust: Finally! Another German!
Mozart: Austria actually
Faust: You speak German. I don't care where you're from
The brits tho have a hard time understanding each other since they speak different dialects. Arthur just bc of his Scottish accent, Will for obvious reasons, and Isaac just speaks science.
When I was in London there were two people in my intern group that both spoke German-Swiss, both from Switzerland, but both spoke different dialects, and so there were times they couldn't understand each other, and that really opened my eyes to how complicated languages are.
Because this means besides the Brothers, and Sebastian and MC, no one is understanding anyone. Sebastian and MC probably know more proper way of speaking French, while Napoleon is more laid back, and Jean is none of those.
Arthur uses words that didn't exist to Isaac so he's like "WHAT ARE YOU SAYING"
Everyone at the mansion probably learns a combination of a laid back-proper French from hearing people speak, and actively learning it.
Napoleon is like "You really don't have to use that word"
And Theo is like "??? What do you mean?? Isn't that how it's said"
And Napoleon and Comte do that a lot to the residents.
Mozart and Faust wouldn't even technically be speaking the same German more than likely.
Then there's Dazai trying to speak Spanish to Leonardo.