Sure they have a wide spectrum of gestures and facial expressions (of course especially apes or gorillas, with which we share a great amount of our genetics), but do they really understand?
Is the record of "Alex" the Grey Parrot impressive?
Yes, but don't forget that a qualified animal researcher and scientist made it her life's project to teach this parrot.
I think that most animals just learn to loosely "say"/do things when it's wanted from them, because of very loose connections and patterns they recognize, not certainly as a way of deep emotional expression.
Maybe someone has Sources for Neurological Studies on this?
Could in my opinion clarify a little bit, what these animals actually "think" while issuing sign language gestures or while talking, therefore maybe unraveling their real capabilities of understanding.