Perhaps this might help:
http://wiki.zimbra.com/index.php?title=Translations
Once you get the language pack installed, all you need to do is change the language settings of your browser and the respective language files will be autmagically loaded by the client.
So, if you're using FF, go to Preferences > Advanced > General > Edit Languages > Select a language to add...
Choose the language you just installed, cross your fingers and reload the browser
