The Multilingual plugin adds its own button to the admin-menu plugin outlet and does not use the add_admin_route API.
If the new admin sidebar is enabled, the admin-menu plugin outlet is not accessible and as such, there is no way to access the Multilingual configuration pages.
To make the multilingual plugin put its menu items where people expect it, I believe we should be using add_admin_route so Discourse core will take care of making it appear where people expect it.