Zenphoto is multi-lingual. There are several aspects of this.
First there is the Zenphoto standard themes, functions, and admin pages. For these you need translation files. We have some volunteers who have been providing same, so you can look at the locales folder to see what language translations have been made. If what you want is not here, someone (you?) will have to do the translations. There is a tutorial and forum for translations should you need to go this route.
Second, custom themes may be made translatable, but of course, no such translations will have been provided with the Zenphoto distribution. See the tutorial/translator's group if you need custom theme translations.
Third, there is individual text strings that are part of your image and album data. For these you will need to enable the `multi-lingual` option. You will then have a place for each Database field for each language you have installed. Of course, you have to provide the translations.
Your users can view the translated pages two ways. By far the prefered is for them to have properly configured browsers which declare their prefered language. Zenphoto will select that language so long as it is one that has translations provided.
If this is not working, you can enable and use the `dynamic-locale` plugin. This will give your viewers a drop-down language selector.