This is part of #378 as it is now a feature included in D8 core (d.org issue):

This was something that tested really well on the D8 UX study, and we should consider adding to Backdrop as well.
User interface changes
- Form validation error messages are displayed below the respective form fields that have triggered them.
- A brief indication of the number of errors, and a link to each error is displayed in the messages area at the top of the page.
Contrib modules
Drupal 7 inline messages module

Drupal 6 (and 7) ife module

Inline Form Errors unofficial and very experimental Backdrop contrib module by @daggerhart:

~~ Advocate for this issue: @klonos ~~
GitHub Issue #:
1040
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)