Multilingual content and single language admin interface
On a fresh install, I could get it working that way:

In admin/config/regional/language/detection


put  Account administration pages on top of Interface text language detection an enable it of course.
enable URL method as second choice
enable Content language detection and put URL on top of detection methods.


 

In user/1/edit language settings :


set Administration pages language with your preferred language


There are various issues on D.O about that, maybe this is the most relevant  one : When content language detection is different from interface language detection, the detected language is not applied to the rendered content. 

Apply these patches :


When content language detection is different ... (patch #24)
'Account administration pages' language negotiation causes 'access denied' in toolbar subtree caching (patch #10)
Admin toolbar should always be rendered in the admin language (if set) (patch #40)


Empty caches :


$ drush cache-rebuild


Language dropdown switcher and everything else seems now to be working fine !

An example of spanish content, strings passed through t() are translated, admin toolbar in english...



The same page edited: admin interface fully in english, but content shows  the correct translation....