I agree with the general sentiment of only your main characters origin should matter, but I don't think only the main character should lead dialogue in SP. I think any character should be able to talk. Though in regards to having another character "switch in", I think that should be able to happen if one of your characters has pet pal and an animal initiates dialogue with your party. It should default to the character with pet pal instead of whoever is in the front, because realistically the person who can't speak to animals would just give the person who can the lead when an animal tries to talk to them.
