I've never quite understood why the Navajo went along with the code talker program.
The US government, the very organization which slaughtered, betrayed, and oppressed their people, came to them with the idea of turning their linguistic heritage into a weapon of war.
Somehow I can't see "patriotism" being the prime motivator, for obvious reasons.
I also can't see self interest or defense as being a convincing argument - neither Japan nor Germany had any real desire or capability to launch a trans-oceanic invasion of North America. (An insanely difficult undertaking which makes me shudder just considering it.)
And even if in some bizarro-world the Axis had been both willing and able to invade, why would the Navajo join the fight on the side of the one government in the entire equation which had committed genocide against their people?
Remember, it wasn't until the end of the war that anyone outside the highest ranks of the Wehrmacht knew that Germany was conducting their own extermination campaign. The worst that could be said about the Third Reich or the Japanese Empire was that they were aggresive, expansionist, militant imperialists - an argument that applied equally well to the US government. If you absolutely had to choose between them, why not choose the side that didn't have a history of betraying and murdering your people?
Or is it possible that they were merely coerced? "Hey, Navajos. You have something we want, and we have the means to take it from you. We can make your lives hell if you don't give it to us - it'd be a shame if conditions on the reservation got worse, you know what I mean? Or if it turned out you were disloyal like those Japanese are, we might have to send you to your own little internment camps. Why make this harder than it has to be, hmm? Why not just do a little favor for us, then come home as war heroes instead?"