I came across an interesting video debunking the idea that Koko the gorilla actually used sign language:
I don’t have the expertise to fully evaluate this on my own, but it accords with the theory which I have heard that it is more plausible that Koko was trained to respond to prompts from trainers than that she expressed her own thoughts and used language in any kind of sophisticated way.
Nevertheless, it remains a fascinating question whether inter-species communication will ever be possible with non-human beings who do seem to use sophisticated communication, such as whales, dolphins, and great apes. The question of whether they are capable is different from the question of whether we know how to help them do it. Maybe one day we will find techniques that allow them to express what they say to each other in a form that people can understand.
P.S. A great recent episode of the Ologies podcast discusses acoustic ecology.