On the Permissions matrix (admin/config/people/permissions) there is a section for Field and a section for Field UI that both contain permissions relating to the User Interface for Fields. Shouldn't the both be under Field or both under Field UI?
If the Administer fields permission comes from the Field module I recommend that we move the Display Mode permission there as well.

GitHub Issue #:
4710
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)