Look at the TRAC system (http://www.zenphoto.org/trac/report/9) for the languages and their current translation state. Also review the translation tutiorial (http://www.zenphoto.org/2008/05/translating-tutorial/). Also join the google group as indicated in this thread.
Then, if the language you wish to translate is not represented, create a ticket for the language and assign the ticket to yourself. If there is already a ticket, post your interest in participating on that ticket. This way all interested parties can work together on the translations.