[D8] Add administration specific language preference

We should add the ability to specify an administrative language that gets used for all administrators. Essentially https://www.drupal.org/project/admin_language module.

Original issue from drupal.org: https://www.drupal.org/node/322995

My site's default language is Finnish but I need to use English for admin pages. There should be next to Default language option the second one where user could choose the admin language. I tried also to change language from my user account but it didn't make any change (bug?).

Changelog: https://www.drupal.org/node/1776768

This patch is actually quite trivial and being from 2012 is almost directly applicable to Backdrop.