As part of Menus are now configuration entities and Menu links have been converted to entities menus and menu links received a langcode property. The menu language can now be configured from the user interface when adding or editing a menu. By default, menu links inherit the language from the menu. Optionally, a separate menu link language can be configured. Additionally, one can optionally show a language selector on the menu link add and edit form, so that each menu link can have the language configured individually.
This is an application of the language select element similar to how it is implemented on content types for nodes or vocabularies for taxonomy terms earlier in New language configuration form element