If you'd like to contribute translations to the VPN software this document explains how.
You can check which translations are already available, and possibly update them if they are out of date:
If you want to contribute a new translation you can take the empty file and translate all strings in there and send it.
The files use the PHP Array syntax.
The "key" is the English string, the value should be the translation.
Once you are done, you can submit your translation file either by creating a "Pull Request" on GitHub, or simply mailing the file with your translations to firstname.lastname@example.org. Don't forget to mention how you want to be credited.