Multiple domains with a language each
You can't stay logged in on a different domain except you'd use some kind of single-sign-on system between your domains.

Cookies are assigned to a specific domain, the browser has no idea that he is still on the same website.

You might be able to configure separate content and interface negotation, so you stay always on your own language, or you could use the admin language feature so a certain user always stays in his language when he's on the site. But not sure if that works 100% for editing translations in Drupal 8, havent tried it yet.