Translating from a found translation

Case situation: let's say that someone posts the lyrics of a song in Russian and wants those lyrics to be translated in German. Can I, if I find the translation into English of these lyrics on a trustworthy site, offer a translation of that found translation into German?
I really believe that the ideal is that people translate from a language they understand. But is it unethical to do it as described above? What do you think?

This doesn't happen very often but I agree with you, this not exactly the kind of translations cucumis is supposed to offer. Finnaly, I think that if the requester is happy with the translation provided, that's the most important. But of course the experts should not give a rating to this kind of translation as it does not reflect the translator ability for the corresponding language.