How gamebreaking would it be to allow talking animals?

[Olgar Shiverstone]

Do it! Then the adventurers need to come across a farm run entirely by animals, where the sheep bleat "Four legs good, two legs bad", and all animals are equal, except the pigs are more equal than others.

 

[Rune]

So far, it has worked out very well.

In that game, the characters are animals, so some other considerations are more significant (but have not been unbalancing); namely, simplifying/eliminating material components (somatic components are fine--ever watch a dog circle three times before going to sleep?) and reworking the stat ranges for animals to something playable.

The ability to talk has never been a problem, whatsoever.