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. See original summary.

Version: 8.3.x-dev » 8.4.x-dev

Drupal 8.3.6 was released on August 2, 2017 and is the final full bugfix release for the Drupal 8.3.x series. Drupal 8.3.x will not receive any further development aside from critical and security fixes. Sites should prepare to update to 8.4.0 on October 4, 2017. (Drupal 8.4.0-alpha1 is available for testing.)

Bug reports should be targeted against the 8.4.x-dev branch from now on, and new development or disruptive changes should be targeted against the 8.5.x-dev branch. For more information see the Drupal 8 minor version schedule and the Allowed changes during the Drupal 8 release cycle.