In the animated movie, Jane asked Tarzan to teach her how to speak gorilla… but how did she know that animals talk if Tarzan never told her? It certainly didn’t SOUND like they were talking, to normal human ears.
If you see two people talking in a foreign language, can you tell they are talking to each other? Like that.
Actually, how did Tarzan know how to tell Jane his name in English if Tarzan has never met humans before? That’s the bigger question.
Maybe in their world they know that animals can communicate in that way, and when she watched him do it she realized. Or maybe she just figured it out through observation.