Showing default language prefix on WP 5.8

[m-hefti]

Hi there,

Since WP 5.8, updating posts or pages with Gutenberg doesn't work anymore, and I figured out it's linked to the "Use prefix for language by default" option. Deactivating this option solves the problem. This bug affects also many translation plugins (Multilanguage by BestWebSoft for instance).

It's due to an 404 error on XHR request "https://your-domain.com/wp-json/wp/v2/pages/83?_locale=user" while trying to save the post or page to the DB. I don't have a clue on what's going wrong, but might it be that the plugin consider "wp-json" as a language prefix and break the request?

Thanks for the amazing plugin and I hope this issue might be solved.