Zoey and I stood [...] lulled by the six bells ringing out over the dusky fields from the church. [...] It is the noblest music, for everyone can share it -- they only have to listen. (2.5)
Music is a language that can be understood by all ears, human and animal. But the bells are also a language of peacetime, so you won't be hearing them again for a long time. This is War Horse, not Church Bell Horse.
When [Albert's father] came back into the stable afterward and began to sweet-talk me and held out a bucket of sweet-smelling oats, I was immediately suspicious. But the oats and my own inquisitiveness overcame my better judgment. (3.11)
It seems like one of the most effective methods of communication for a horse is bribery. Even though Joey doesn't trust Albert's perpetually inebriated father, all it takes is one mouthful of yummy oats and they're total BFFs.
With Albert riding me, there was no hanging on the reins, no jerking on the bit in my mouth; a gentle squeeze with the knees and a touch with his heels was enough to tell me what he wanted of me. I think he could have ridden even without that, so well did we come to understand each other. (3.1)
Albert and Joey develop a complex method of non-verbal communication. Joey seems to think they have an almost telepathic connection. And as Joey learns, not all riders can do this. He and Albert have a special kind of bromance.
"If you can understand anything of what I said, then pray for her to whatever horse god you pray to." (10.9)
Plenty of people talk to Joey, even though they have no idea if he can understand them. Emilie's grandpapa seems to do it out of desperation. The horses brought happiness to Emile's fragile life, so maybe they can save it in the process.
[Emilie] did not need to lead us. She only had to call and we followed. (11.1)
In this case, the language used to communicate with horses is one of mutual admiration and respect. We could probably all take a cue or two from Emilie.
[Topthorn] lay where he was, breathing heavily, and lifted his head once to look at me. It was an appeal for help—I could see it in his eyes. (14.5)
Even though War Horse is told from a horse's point of view, the horses don't communicate with each other in any sort of secret language. All Topthorn can do is plead with his eyes, the same way Joey might do to a human if he were in pain. Would Topthorn have survived if he could talk?
"I can't speak a word of German, and I can see you can't understand what the hell I'm talking about, can you?" (16.6)
Humans and horses aren't the only ones with communication issues. The English and the Germans can barely communicate, either. But these men are able to get along with each other just fine, if only for a bit—maybe they've learned something from the horses.
I reared up on my back legs and cried out to him to recognize me. (17.5)
Joey does his best to communicate with his human, Albert. Here, you can practically hear him saying, "Hello, Wilbur!" Oh—we mean Albert. Unfortunately, Albert doesn't speak horse, so he doesn't recognize Joey until they clean him off.
I felt a sudden looseness in my throat and neck, so much so that I could call out, albeit softly, for the first time. (18.25)
Here, Joey is saying "I'm feeling better!" This call is one Albert does understand, but only because Joey had been silent for weeks. At that point, any noise is good noise.
"I can't tell you any more cause I know you'd understand every word I said, and then you'd only worry yourself sick with." (20.6)
Albert is one of the few characters who believes that Joey can understand him. But wait a second. Albert and the other English blab the whole thing about the horse sale within earshot, and Joey doesn't actually react. So maybe he doesn't understand, after all. What do you think?