War Horse Theme of Language and Communication

You might think that communication would be an issue for Joey since, you know, he can't talk. But in War Horse, both the English and German—who speak two different languages—manage to communicate just fine with our horse friend. It's all about tone and body language for Joey, and for a brief moment in time, he's even able to brief the two sides together without conflict or violence. So do horses speak the language of peace?

1. Why do so many humans talk to Joey even though he can't talk back?
2. How does Joey communicate with other horses, and how does he communicate with humans? 
3. Why don't the horses in the book talk to each other? How would the story be different if they did?