The Japanese tried to crack the code by torturing a poor guy called Joe Kieyoomia who was Navajo, but not a code-talker or even aware of the existence of the program, but they didn't get far. Amazingly, he survived the concentration camp, the Bataan death march and getting nuked in Nagasaki, returning to the US and living until 77.
https://en.wikipedia.org/wiki/Joe_Kieyoomia