Messy and scrawly! It's perfectly legible, but he writes really fast and gets excited, so it ends up being some messy combination between cursive and print. He's long since figured out the translation thing, but is used to writing in Japanese by now so he switches between French and Japanese in a way that would be very confusing to anyone who wasn't him if it wasn't translated. His handwriting does suddenly become a bit more clear and formal when he switches to Japanese, because he has to think about it a bit more and it comes less naturally. So if someone was aware of it, they could probably tell when he switches languages half way through a sentence because he wants to use a Japanese/French word when he was otherwise writing in the other language.
Sometimes he draws cute little illustrations too, just because.