The correct answer to this open question is the following.
The Navajo is considered to be the largest Athabascan language group in the United States because its main language, Diné, is spoken by nearly 150 000 to 170 000 Navajo people, mostly in the northern parts of Arizona, and some regions of Utah, and New Mexico. As the tonal language it is, Navajo is extremely complex to speak and understand if you are not part of the tribe. So complex that the United States Navy used the Navajo language during World War II as a code language in battles against the Japanese troops, in what was known as the Pacific theater of the war.