Error REST wp-json with WPML

Ok, that sounds like a problem best raised in WPML support.

I’m not sure, what filter are you using for getting that endpoint?

Gregorio

The plugin uses the standard WordPress function get_rest_url

The fact is that /en/wp-json/ works, for example: https://www.fondazionelia.org/en/wp-json/

So why does the plugin say rest_no_route?

It may work if you view it, but that doesn’t necessarily mean the same when a plugin tries to use it, particularly if using a POST.