I'm wondering this in light of the prevailing stereotype that people living in the United States tend to be monolingual and ignorant of other cultures. By contrast, many people living in Africa, Europe, and Asia grow up speaking three or even more languages fluently.

Well, what about the Native Americans? They were frequently in contact with other tribes, sure. These different tribes probably also spoke different languages. But did that mean they were necessarily multilingual?

Since geography affects the audience you're able to communicate with, I'm guessing that neighboring tribes tended to share linguistic roots and were therefore similar (kind of like Spanish and Portuguese). Were there Native Americans who were in contact with tribes that spoke completely unrelated languages, yet this did not pose a language barrier to either party? Or was one language all you really needed to know to get by in life?