The language of menus and menu links can now be configured from the user interface

As part of Menus are now configuration entities and Menu links have been converted to plugins with one plugin implementation being a content entity. Thus, menus and menu link content entities received a langcode property.

One can optionally enable a language selector on the menu link content add and edit form, so that each menu link can have the language configured individually and a translation can be added for each configured language.

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