Many dogs can understand almost every word humans say, while humans seldom learn to recognize more than half a dozen barks, if that. And barks are only a small part of the dog language. A wagging tail can mean so many things. Humans know that it means a dog is pleased, but not what a dog is saying about his pleasedness.
Stories of the Hundred and One Dalmatians (ed. 1989) - ISBN: 9781851520763