Apparently the latest update (to 1.13.3) ruined the ability to install backdrop in another language. It worked ok in previous version.
I am using Danish.
There is no other language other than english no matter what I do before install. Installing Danish .po file (etc.) after install doesn't worh either . nothing gets translated (yes I clear cache etc.).
please go to admin/config/regional/language and disable "English", then clear caches.
Problem seems like you have two language site without properly set up languages management.