Penny asks Koko a question and Koko ponders the answer. Why isn”t Penny using sign language? Because Koko understands even more spoken English than she does American Sign Language (2000+ words vs 1000+ words).
When Koko was a young student, Penny and staff would almost always speak simultaneously as they signed to her. This turned out to be a very good idea, as Koko can now understand more people than can understand her (signing), which empowers her as an ambassador for her species.
Perhaps she will learn some new languages as we reach across the globe via internet video-conferencing. (See “Interspecies Communication Research” on Koko”s WishList.)