In the final scene of the movie as Dunbar (Kevin Costner) is preparing to leave, Kicking Bird (Graham Greene) is searching for him with a pipe he had made. When they meet, they exchange pipes and speak to each other in perfect english! (American rather). Remember, this movie was unique as all the Native actors learned to speak Lakota for realism. There is no indication that any of the Natives learned english and this is the only time that one uses it.

Dunbar taught Kicking bird English in the beginning of the movie.