We rely on http://translatewiki.net to handle translation for us, so

1. sign up at http://translatewiki.net
2. Visit our TWN project page
3. Click on Translate Etherpad lite interface
4. Choose a target language, you'd like to translate our interface to, and hit Fetch
5. Start translating!

Translations will be send back to us regularly and will eventually appear in the next release. Cool, eh?!