The code talkers were only in action late in the war, and as noted what they transmitted was very short-lived tactical messages. Suppose the Japanese managed to scrape up someone who spoke Navajo - say, a returned immigrant who had lived in the Four Corners area. Then he has to teach Navajo to other Japanese, and then... they have to learn how to read the code talkers' messages, which besides being in Navajo used a lot of code words. It wouldn't be that hard to "break" that code...
But also, any Japanese in position to listen to American tactical radio chatter was probably on an island the US was taking, meaning soon to be dead. So it just doesn't seem practical.