There were footnotes I've seen on Navajo topics that they tried Satsuma(Kagoshima) dialect for the same purpose, but there were no shortages of speakers at the

There were footnotes I've seen on Navajo topics that they tried Satsuma(Kagoshima) dialect for the same purpose, but there were no shortages of speakers at the Allied side and the use ended quickly. Uniqueness of Navajo and other Native American languages was that they were all complete multi millennia isolate from English thanks to how USA came to be. Japan wasn't like that so dialects aren't divergent enough for this use case. Ainu might be a candidate but it's way too high profile.