I will also be doing the 6-hour study guide tour. From what I understand, you move along with the group through different sections and not necessarily return to same spot to retrieve someone. The tour stops by the entrance for a 30 minute break between Auschwitz I and Birkenau after about 3 hours, I think. Then the group takes a shuttle bus to Birkenau.
Does your dad have mobility issues? I wouldn't recommend the 6 hour tour if so. And both tours involve a lot of walking/covering a wide area. If your concern is just the language, I know there are apps on the web that I looked at when I was considering going with unguided option (but then realized my time would be shorter). As long as he is with the group, he could listen to his own guide in his native language.
Is there an option for the shorter tour (3.5 hour) for his native language? Do you speak it so you could then join a different tour? Or would the shorter tour, even English, be a better option if he has to wait for you? All tours stop by the entrance/cafeteria between camps. This is from what I read and researched online - I'll be there in September.