Most of the Native Americans in westerns are Navahos. When the script calls for them to say something in a Native American language, the director would tell them, "Just say something in Navaho, after all, who speaks Navaho?"
I think we all know the answer to that one.
According to the anthropology professor who told me this story, an actor playing Cochise (who wasn't a Navaho.) brought the house down at a reservation cinema, when he charged the cavalry shouting, "Hello friend! Good to see you!"

The majority of the people who see "Robin Hood" won't know a Yorkshire accent, anymore than they can speak Navaho.