What is little known is that the Japanese had a Navajo POW (probaly a NM National Guard member since these were in the PHilippines, but I'm not sure).
They tortured the POW to translate, and he couldn't, since although he understood the words, they were gibbrish.
The code was not used in major transmissions, since if I remember correctly, there were 2 or 3 German professor linguists who could translate if given enough time.
And other lanugages were used to a lesser degree: Choctow and Commanche, I believe.,
Linguistically, Navajo is "tonal". The vowels have different lengths in different words. There are "glottal stops", and there are several consenants impossible for Europeans to say. Finally, there are many irregular verbs and different tenses.
I only know two phrases in Navajo: Ya'a ta he (hello, or goodbye, literally things are good) and "nez gaish" (where does it hurt?)