Why were the japanese unable to decipher the messages of the navajo code talkers?

Firstly, the Navajo language is not written, it is only spoken by the Navajo tribe. So the only way the Japanese could have broken the code is if they knew Navajo themselves. Also, a lot of words didn't even exist in Navajo and had to create new words. Because of this, orders and commands could be broadcasted over radio for easy communication without fear that the enemy could understand them

Andrei karlov was the russian ambassador