Answer: Navajo language is a super rare language. Basically, since not many people speak it and understand it, it is great to use it as a military code language.
Explanation: Using the language of the Navajo Nation was a brilliant strategy used in World War 2. As its grammar is very different from Japanese and German (the enemies of the time), the U.S military hired Navajo speakers to use their language to transmit top-secret codes and information. And it worked. It served its purpose as a military code language.