It wasn’t so much what words they used. The Japanese just didn’t have any way of translating / understanding Navajo.

Kind of like you may hear a language, and recognize what it is, but no idea what they are saying.

The Navajo language was not written at the time, and it’s very different than others and subtle differences in tone can make all the difference. Their written notes of what they heard were probably generally inaccurate which made breaking the code impossible (vs enigma which was exactly copied, and the output is German, they just then needed to break it)