The implementation has been released in [1969] and will show up in tonight's nightly build. To activate, find the "multi-lingual' checkbox next to the language selector on the admin gallery configuration tab. Check the box and you will have fields to enter translations for any of the zp supported languages. They do not all have to be populated. There is a fallback default whick will select the user's language if it exists falling back to the language specified by the language selector, falling back to the en_US string. (If none of these exist it will select the first non empty string.)
Please report problems through the ticket system so we can keep tabs on them. Remember that this is a brand new feature. There may be some things that don't quite work right. Let us know.
Visit http://www.zenphoto.org/trac/browser/trunk. At the bottom of the page is a link to download a zip archive of the revision. Revisions keep being made. This link is to the most current development release of zenphoto. You can also get what we call the "nightly" build. Visit this link http://www.zenphoto.org/files/nightly/ and pick the most recent.
Generally it is a better choice to use the nightly builds. In your case that is my recommendaton. All the language changes are in the build from 2008-07-20.