Problem/Motivation
If the site has an admin language preference option set in negotiation, users can set their admin language preference. However, even if that is set, only admin pages are affected. The admin toolbar is not. The admin toolbar displayed on non-admin pages will use the language of that page, not the admin language.
Proposed resolution
Negotiate an admin language for the toolbar and use that to render the menu.
Remaining tasks
Do it.
User interface changes
The admin toolbar will be in an admin-appropriate language.
API changes
Likely none.