I am facing a problem on using a language other than English by default.
I chose French as the default language when installing Drupal 8.
And all went very well until I activate "Enable interface translation to English" in "admin/config/regional/language/edit/en" to be able to modify some translations.
I have add and modify translations in English, it works very well
But by now if a new user registers on the site in the English pages, All the notifications sent by mail are in French !!!
I did a test by adding the translation of the title of the mail of the inscription:

msgid "Account details for [user:display-name] at [site:name] (pending admin approval)"
msgstr "Account details for [user:display-name] at [site:name] (pending admin approval)"

And it worked, but I do not see myself adding all the English translations, that will be at 99% the same strings.
And logically, there is no en.po on localize.drupal.org.

Is there a solution to this problem?

Test on Drupal core 8.3.3 on fresh install without any contrib modules / PHP7.0.20


steveoriol created an issue.