Human language is also trained behavior. The question is how complex the cognitive nuances of communication can develop. Your dog probably likes to play, but he has no way to communicate the desire to play with sign language because he doesn't have fingers and the ability to coordinate them into legible symbols. A dog can bring its leash when it wants to go out, or bring a ball when it wants to fetch, etc
Animals are not stupid. They know what they want and they figure out how to communicate it with humans, assuming the humans are smart enough to figure out what they are trying to say.
Your point seems to be that Koko's language abilities weren't exceptional and I agree, but neither are humans' We just happen to have more nuanced hearing and vocal abilities that Gorillas either don't have, or their auditory language is too different from ours for us to communicate with each other across the species gap.
Do you listen to song birds? Song birds have developed the ability to communicate over long distances as part of evolving the ability to fly far. If they wouldn't have developed this ability, they wouldn't have been able to search as far and wide for food to feed their young. So we can guess that their songs probably identify them to their mates so they can communicate with the nest and for courting, but what else might they be communicating? Maybe the one in the nest makes a special call when she wants more of one particular type of food or another. That wouldn't be surprising if 1) their minds are able to distinguish between the different types of food, 2) their minds can distinguish the different sounds, and 3) their minds can associate each sound with its respective food.
Communication is not rocket science, but it requires some intelligence, and it is interesting to study how each species uses its particular evolved skills, such as fine motor control of fingers to communicate with other beings who couldn't otherwise understand what they want to tell.