dubber

We are making a certificate for volunteers who worked at our film festival, it says: " This is to thank and certify that ........ worked as "dubber" during Dialektus festival."
Is these a better expression for one who reads the translation of a film while showing?

Do you mean that someone sits and reads a translation of what the speakers are saying live while the film is showing? I think this may be done in some countries (I believe the actual soundtrack can still be heard, so it is not like usual studio dubbing), but I'm not sure this is a common process in English, so there may not be a handy term for it.

I think in the movie industry they are called "dubbing artists" or "voice actors", but I don't know whether these would fit your particular description.

I usually think of "dubbers" as people who dub over the original voices ahead of time. I might thank these people for "providing Hungarian (or whatever) voiceovers", but I'm not an expert, so maybe 'dubber' would be fine.