All of Frank's characters in Treasure Island were either performed directly by him or dubbed later, so the credit attributed to him is accurate. The credits don't bother to mention when someone else actually had the puppet on their arm, because dubbing is done so frequently in TV/film puppetry anyway (for example, in a scene with both Kermit and Rowlf, the puppeteers would have to lip synch to Jim's pre-recorded lines since he couldn't perform both characters at the same time).
Bill just performs the Chef's head and voice, just like Jim, and someone else (I'm guessing Brian, but I'm not sure) performs the hands, just like Frank used to. It would be too hard to coordinate otherwise, if Bill was performing the head and ONE hand while another puppeteer provided the other hand!
The dialogue in Swedish is actually written out in the script, so the performers don't have to learn it. They just have to be good at imitating Jim's vocal performance of the character.
In a way, it does seem kind of sad that Frank isn't as involved as he used to be. But two good things about the replacements: 1) Frank's characters are around with greater frequency now. 2) The chemistry is better. Frank never quite got into playing his characters opposite performers other than Jim. From what little I've seen of Ernie/Bert and Kermit/Piggy now, Steve Whitmire and Eric Jacobson have much better chemistry than Steve and Frank. The Christmas movie, I think, will be the ultimate test.
If your college does offer a class in Swedish, let me know!