Description of the need
If you ever set the Default picture on admin/config/people/settings, then you could have an impression the settings in the next section for Picture display style would apply to the default picture.

Unfortunately, it's not true as the HTML-code always shows:
<img src="http://in.docksal.site/files/pictures/default_image.png" alt="Backdrop CMS's picture" title="Backdrop CMS's picture">
for the default picture. However, if a user uploads his/her own picture, then the image style kicks in:
<img src="http://in.docksal.site/files/styles/thumbnail/public/pictures/picture-1-1614678151.png" alt="Backdrop CMS's picture" title="Backdrop CMS's picture">
Proposed solution
I don't see any reasons for the style selected in Picture display style section not to be applied for also the default picture. If you, guys, support this feature request, then I could offer a respective PR.
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)