Tim May comments:

According to an episode of "The X Files," which dealt with Navajo code-talkers, the answer is that young Navajo men are losing their fluency in Navajo, especially of the nuances and double entendres that code-talkers relied upon. (For those who scoff at using a television show as a source, writers for shows like this often do more interesting research than, say, the average encyclopedia article will report.)

Navajo also has something now it didn't have before WWII, a written language, and I believe a rudimentary dictionary. Brian