You can definitely add more languages.
1. Check out the configuration guide: Multi-Language Support for Proactive Translations section. It will tell you how to activate them in the database.
2. Once activated, you will need to assign them to the users that can add translations. You do this in UGM on the user profile.
3. Once languages have been added to the user, the user will see the language options in the proactive language popup.
Hope this helps. Let me know if you have any further questions.
Just as a reference, in addition to our National Language Support packs we support the following languages for proactive translations: