Update: Plans in Drupal-land have shifted towards Create accessible markup for a drag & drop file upload widget (and ensure there is an accessible alternative interaction) and the original issue slated for D8 has been closed in the meantime.
Original request
This is a follow-up to #1380, the respective issue for https://www.drupal.org/node/2113931 and part of #779
Here's what they ~are~ were aiming for in D8:

GitHub Issue #:
1445
Recent comments
There is a Drupal 7 contrib module that "lets the administrator see all administration pages in her preferred language" and which could be ported to Backdrop: https://www.drupal.org/project/...
Allow admin to select admin language seperate from front end language (multilingual)
@stpaultim – You're right: my approach affects also the main menu. I guess, because menus are also considered as user interface (not as content). @findlabnet – If I didn't miss anything,...
Allow admin to select admin language seperate from front end language (multilingual)
Go to the account edit of the desired user. On the horizontal tab below "Region and Language," select "English" or another language. WFM.
Allow admin to select admin language seperate from front end language (multilingual)