Hate to break it to you, but the First Party Mario and so on Games are always identical worldwide, every version is multilingual. Or at least Mario Odyssey was, Mario Kart 8 was, Tennis Aces was. Reckon it'll be very much the same for Smash. So as long as your Switch is set to French, the game will run in French - regardless whether you pop in a European or a US copy of the game. Might as well get your game locally and set your Switch to English.
I'd recommend to wait the release of the game to see if there isn't a Euro copy that will cost less to you with english language on it.
Probably to play the game in english it will require you to run the Switch with it's language set to english though.