What is the ownership of the other directories in /usr/lib/python3/dist-packages/mailman/? They are probably also owned by root and that should be OK as long as they are world readable.
I have checked the permissions and properties of the it directory. They are the same as the en directory.
Please tell me what messages. Do you mean if I subscribe to the list, I get a welcome message in English, but the list owner gets the notification of a new member in Italian?
Not exactly. If you subscribe a user to a list, the Welcome email arrives in English. On the other hand, if an admin is registered for a list, he receives the email in Italian. But I solved it by changing the value "en" with "it" in the mailman3 database, the mailinglist table, the preferred_language column. Now those who subscribe to the list receive a Welcome email in Italian.
I think you need to set the system default_language. In mailman.cfg, set [mailman] default_language: it
I had changed this value from the beginning.
Thank you very much for your valuable advice and patience
Max