There are so many amazing studies on this. They actually used for the most part Native translators. It was the natives that often spoke more than one language because of the tribes always being so near to each other and consistently having to make alliances as well as war.

La Malinche is a great example of this. Although her origins remain murky she is suspected of speaking 3 native languages, Chontal Maya, Yucatec Maya, and Nahuatl, as well as Spanish. By being exposed to only new languages one will be forced to learn them. This happened to La Malinche when she was put into Cortez’ hands.

A common misconception is that the Spanish had no native help, when in reality Cortez used the already messed up relationships between the native groups to take down the Aztec Empire, which had conquered them. While there were Spanish interpreters, many were also native.