Hi Alessandro. I have the same kind of issue here. I use a purchased theme called Axioma. With this theme I can create multiple Portfolio items. But the translation is not working on the front-end for them. It’s always displaying the default version. I can enter the url like that /fr/url-2 and it’s working, but /fr/url is showing me the default one (EN). I can send you the link of the website in private message.
And by the way, it’s a really great plugin !
Thanks.