And in it the author (who spent decades working with and teaching chimps) concludes that chimps have the intelligence to talk but not the vocal chords. They can understand humans talking (in the same way that a dog can understand verbal commands) but they cannot do much besides grunt or hoot back.
However when you change the conversation method to something else (such as sign languge) chimps can talk back exceedingly well. In the book mentioned above the author has video proof that the chimps can talk to humans (and each other) at the level of a two to four year old child.
This is the main chimp in the book:
This doesn't seem to just apply to chimps either. Gorillas (and other large apes) can do the same.