Good evening Davide and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
I would love to help you with this topic.
What is going on is that at the moment of writing, Polylang Pro is compatible only with the Events Calendar free.
Officially the Events Calendar Pro compatibility has not been achieved. Especially when it comes to recurring events.
In fact official support for Polylang with the Events Calendar is fairly recent.
I chose the word officially because I know some of our customers have been able to achieve a great deal of compatibility using different tweaks, including compatibility with Event Tickets and Event Tickets Plus.
So I am afraid there is little more I can offer at this point.
I would recommend reaching out to the Polylang team about this and perhaps checking out the forums of other Polylang Pro users to see if there is not a possible workaround.
I wish I had a better answer for you, but for now it’s the best one I have unfortunately.
Have a great day!
Geoff B.