Bonjour Alexandre,
from what I understand, you want to only show the scene for the selected language within the course's menu.
Unfortunately, I believe Storyline does not allow modifications of the menu while the training is being worked on.
What you could do is closing both scenes within the menu (just open the "menu" tab in the "player" settings and klick on the small arrows next to the scenes' names in the menu). Then, you could open the "Advanced Options" menu and set the navigation restrictions for the menu to "restricted". That way, your learners could see both language scenes in the menu, but only access previously visited slides, which will all be from the language scene they've been working on up to that point.
Regards,
Marc