Can't translate a content into a language different than that of the administration pages language setting

I've found a problem translating contents into another language in a particular scenario. Those are the steps to reproduce the problem (tested on simplytest.me):

1.- New Drupal 8.2.5 installation in Spanish with the standard profile.
2.- Enable Content Translation module.
3.- Set a content type to be translatable, for example, the page content type.
4.- Add the English language.
5.- Go to language detection and selection settings (admin/config/regional/language/detection) and enable "Account administration pages", "URL" and "Browser" detection methods, with that exact order.

6.- Go to admin account settings (user/1/edit) and set "Administration pages language" to Spanish.

With that configuration, if you create a new page node in Spanish, then, when you go to translation page for that node and adds an English translation, a locale combo appears in translation edit, in which the only possible value is Spanish. If you save the translation it seems that it creates a redundant translation into Spanish. If you navigate to /en/node/1 it appears in Spanish. Neither you can't edit the English translation.

If you still have an existing translation, it results in data loss for the translated content.

As a temporary solution, I've solved the problem configuring the "URL" detection method above "Account administration pages" detection method, but the administration pages language setting, obviously, has no effect.