[delayclose]

Having no native teachers available increased the difficulty, and the WWII usage was against non-native English speakers. That said, the Native American languages that I know of are grammatically quite different from English.

[oscardelben]

In that specific case there were less than 30 people who knew that language in the US during WWII, and the navajo language was also completely different from other native languages.

UPDATE: I mean 30 people outside the navajo tribe.

[eru]

And they added a (simple) coding system on top.