Always keep admin UI in English when translating content
This functionality is partially achievable since Drupal 8.6.4.


Navigate to Detection and selection
(admin/config/regional/language/detection)
Adjust your settings accordingly. Be sure to activate "Account administration pages". Your content language detection methods are optional.


Edit your user profile and set your "Administration pages language" to English.


Now your Admin UI will stay in English no matter what the other language detection methods resolve to. That means while creating a translation, the interface will remain in English. 

However, the unsupported part is keeping the Admin Toolbar in English when being redirected to the translated page on saving it. But when going back to the Admin UI it's in English again.

These issues are dealing with that:
https://www.drupal.org/project/drupal/issues/2313309

And if you use the Admin Toolbar module:
https://www.drupal.org/project/admin_toolbar/issues/3004575