Problem/motivation
Menu items currently are all in one bundle called "Menu link". The language subsystem has a standard language setup process for language settings and inheritance (whether language selectors are shown on content entities, etc.) which is bundle based. So long as menus only have one single bundle, you cannot make certain menus multilingual while others are single language and you cannot hide/show language selectors per menu and you cannot make menu items translatable on a per-menu basis. You can only set these settings globally for all menus. This is not very realistic for multilingual sites.
Proposal
Introduce bundles on a per-menu basis.