Admin bar retains site language in front end

[joedolson]

If you've selected a user language, the admin bar is only in your selected language while you're in the admin.

Steps to reproduce:

1) Add at least one additional language to system.
2) Go to User Profile and select a different language
3) Visit site front end; admin bar will be in the system language instead of your user selected language.

[davidakennedy]

@joedolson and I tested this together at WCUS Contributor day. I first noticed it because when using the RTL Tester plugin for theme testing. There seemed to be an issue when switching to a RTL language. The Admin bar on the front end would not get the proper styles. We suspected this could be related to user admin languages, and tested.

[swissspidy]

Duplicate of #38643.

That's currently by intention, as a few bugs need to be resolved first.

See #26511 and especially #29783 for discussions and background.