Problem
Drupal understands the language of configuration in Drupal 8, and translation to other languages (as well as possible other contributed functionality) depends on the language of configuration. Configuration entities like menus, vocabularies, views, etc. have native language support. Views got a language selector in #1935022: Add a language selector on views and vocabularies got it even earlier. Menus are in the same boat, and could use a language selector on creation defaulted to the site default language.
Proposal
- Add a #type => language_select element on menu config entities.
- Add tests ensuring changing the language worked, saving back to the config entity storage.