Every animal has its own language or rather a particular sound through which it communicates.We say only humans speak because only our language can be identified and understood by us.Just as we can't speak like a dog,cat or pigeon ; similarly they can't speak our language or in other words talk. There is no such thing like only humans can talk.The only difference is that humans only understand their ways of communication and animals understand theirs.
Hope i helped u :)
