System emails translation
Here is the steps to get user email in your language (describing all the steps in case you missed any)

Default installation of Drupal 8

Enable translation module (core modules)


Configuration Translation
Content Translation
Interface Translation 
Language


add your language visiting

/admin/config/regional/language
in my example added Greek with Language code el

translate emails visiting

admin/config/people/accounts/translate
and pressing edit on your language 
which in my example would be take me to this page
admin/config/people/accounts/translate/el/edit

add language switcher block (optional)

so you can switch to appropriate language

visit registration form for your language

in my case /el/user/register
and fill out the form 

The email should come in the registration language 

It came in Greek in my test.