Multilingual menus: logic and architecture
It seems to be impossibe to have one multilingual custom menu in Drupal 8.0.

What you have to do is to create one menu per language. You specify the one language of the menu as you show in your image.

Then in blocks you place all your menus in the same place. Here you can choose which menu is chosen for which language.

This is far from elegant, since you have to edit each language separately without a translate button. But apart from being a cluttered process to create, it works nicely.