How to show registration page in another language, maintaining its natural behaviour?
Navigate to /admin/config/regional/language within your site, and verify these things (and correct where appropriate):


Use "English" as your default language
use the "Edit" link for "English" on that page, which will bring you to "/admin/config/regional/language/edit/en". On that page, make sure the value for "Path prefix language code" is "blank".