Leaving here, if anyone should stumble on the same problem.
1) downoad the phar file from osticket
2) create a subdir in the include/i18n directory with the ISO code of interest (for me it was it_IT)
3) uncompress the phar file
3) copy the uncompressed phar content in the subdir created as per 2
4) chown as needed the subdir at hand
After that, if your language pack includes the email templates you can use them as you see fit.
HTH someone 🙂