With CatKeysEditor you can translate the plain text catalog files used to localize Haiku applications.
Simply get the 'en.catkeys' of the application to be localized and rename it using the standard two-letter language code of your target, e.g. 'fr.catkeys'. Then open it with CatKeysEditor and start translating.
When you're done, you have to recompile the app making use of the new catkeys.