If you killed the Bishop and took his real balls to the Prince of Canada as proof then you probably didn't get him as a friend. I spared him and tricked the prince with dire pig balls, getting the bishop as a friend. The guy in the cave is from Quebec and he's in the cave as a sort of imprisonment because he can somehow get in the way of the prince in some way (prince is up to no good). It's not something really explored because you're just there to get the French writing translated.
But anyway this is just my own guesses, I didn't see what happened.
I've had those moments when I do a campaign long achievement but don't get it for some weird reason, frustrating as hell.