Based on the History of Native Americans there were over 300 languages spoken throughout history.
If you are asking what the most common one is today, it would be Navajo because around 2/3 of all households that still speak native american languages speak Navajo.
However, if you are looking for one common language they all spoke during any part of history. It has been known that most of the tribe leaders or head of 'communication' in that tribe learned english during the American Revolution. This was to make trades with the American 'white man'. More often the Indians would learn english to trade rather than the 'white man' learning their language because it was just easier for everyone.
