The code talkers used the Navajo's native language as it was already a "set" language and, with people in the US armed forces who have Navajo background and know how to talk it, well, you didn't have to train people to learn the code. This solved two problems, in which #1, already a group of people already knew how to speak it, which means less training, money spent on training, etc, and #2, the Japanese were baffled by how the US could use a native american language as a code, which meant that the Japanese couldn't break it, because it had no relations to other writings
The correct answer to this open question is the following.
The code talkers developed the original version of the Navajo code in that they were looking for a special code to use in the war that was extremely difficult to be intercepted and translated by the enemy. That is why the code talkers developed a special code with elements of the Navajo language. During World War 1, Choctaw transmitted codified messages to the US Army headquarters in countries like France. Years later, by the 1940s during World War II, the US Army recruited Native American Indians such as Cherokees, Comanches, and Hopis. But in 1941, it was the Marine Corps the ones that hired Navajo Code Talkers to transmit secret communication during the war.
