If I install D7 in a foreign language, the english interface is installed too. If I add the module "admin_language", I can decide, that all admin pages and the toolbar menue are in english. That is very good to have the same admin language, if I want to follow an issue or an doc on d.o.
If I install D8 in a foreign language, the "Administration pages language" switch on my "user/1/edit" page only knows the installed foreign language. I cannot select "English".
At the moment I have to install english too in a second step. Then I can choose "English" as my "Administration pages language".
My idea is to add a checkbox (default: checked) in the D8 installer to enable the english translation, if the user wants to install a website in a foreign language.