The USMC risked a very tiny chance that there was any linguistics professor in Imperial Japan who might have some dim working documentation of Navajo in the one dusty library book on Navajo ever published, but they would have had to be a Navajo on the reservation to understand their familiar banter, and then decipher the code they were speaking. Layers of complexity kept this valuable military resource safe from the enemy.
A Navajo US Army soldier was even captured and identified by the Japanese at Bataan then tortured into attempting to decipher Navajo Code Talker messages that he couldn't understand because it was in code. There's just no flippin' way that anyone would have had a hope of cracking the Navajo Code Talker's language. Just forget it.
Indeed.
Using the Navajo codes was a stroke of genius. Bless those who served.