Good evening Riccardo and welcome back!
Thank you for reaching out to us.
I would love to help you with this topic.
The proper place to put your customized language file is in /wp-content/languages/plugins/
This will make sure that future updates will not overwrite your work.
However, you might need to update those custom language files from the pot file when we come up with plugin updates.
Additionally, if you want to make your translation public, any help is always welcome here: http://translations.theeventscalendar.com/projects
Let me know if that helps.
Have a great day!
Geoff B.