Question

What two traits made the Navajo language a good choice for a secret code?

Answer 1

The language could not be recognized by the Japanese and it let information  be spread threw to Americans without the Japanese being able to find out what they are saying.

Answer 2

1 would be that the Navajo language is extremely complex, and was hard for people to learn let alone be able to crack the code.

2. Very few people know how to speak Navajo plus it was not written.

I am part Navajo so I hope I was able to help :)