Thank you Chuck! But after looking into phrases.php I think it's easier to take the build-in translation system. There I can see immediatly which effect it takes. And also I can see what to be translate (not all phrases are neccessary for the users).
btw: Maybe I am ready with it until Classifieds 5.0 :-) .... is it possible that other users can work with an SQL export file of the translated phrases?