Drupal - How can I have admin language English and frontend language another?

First go to Administration - Configuration - Regional and language - Languages - Detection and selection tab and make sure to have “Account administration pages“ enabled.

Then go to People and edit your user. Go down to the Language settings section. You should now see a Administration pages language dropdown. Select and save.

This module has been included with Drupal 8 core. Refer to this issue for more information.

That information is shown in the blue box on the page that you linked to.

The linked page has also information on how to configure this in Drupal 8.