All "administration pages" in English and admin menu always LTR?
After you change default site language to Farsi, and admin pages language to English, go to your user:admin page and change the admin's language to English as well to bring it in accordance with the management backend language (English).

Maybe flushing of cache will be needed; In any case if it didn't work then it's a bug and should be reported at Drupal.org ...