I always theorised there was some extra-auditory or human-inaudible cues in Ubese (Boushh) that filled in the gaps. After all, the only individuals responding directly were non-human: Jabba (who probably could understand, but choose to use C-3PO for egomaniacal and/or ceremonial reasons) and of course old Goldenrod himself.

Hey, my linguistics degree had to be good for something.