I added a RTL language translation file (e.g. Arabic, Persian, Farsi) to my Drupal 8 site. I want all the administration menus to always be in English and LTR. On the other hand, I want the front-end pages of my site to be in another language (e.g. Persian, Farsi).
I did the following.
I enabled the Language module, and set up one additional language. Installation was done in English; I added Persian.
I configured the administration language using the following settings.
Account administration pages: enabled
Admin >> Edit Profile >> Edit
This works properly for all the administration pages. Those are in English and LTR except the front page, user/1/, and admin/structure/block/demo/[theme name] where the administration menu is in Persian and RTL.