The only Native American language in the United States spoken by 100% of its tribal members is Havaupai. It is a recognized dialect of Havasupai–Hualapai spoken by approximately 639 people on the Havasupai Indian Reservation at the bottom of the Grand Canyon. To put this in context, there are 573 Tribal entities recognized by the federal government (and more that exist but are unrecognized). Of these, the Havaupai is the only tribe where any member can speak to any other member in the language of their ancestors.
Here is why the the loss of language is genocide. The forcible changing of children from belonging to one group to belonging to another, whether through education, adoption, or orphanages, is part of the international legally recognized definition of genocide. The loss of Native American language due to schooling in English is therefore within the realm of cultural genocide.
