Question

The U.S. military first experimented with Native American languages in military intelligence

Answer 1

The correct answer here is the World War I.
Maybe the most recognizable and most famous code talkers in the US military history were the Navajo Code Talkers which operated during the World War II but this was actually pioneered in the World War I where we see the first instances of experimenting with the Native American Languages as a source for military code. In the World War I, Cherokee and Choctaw peoples were the ones to do this.

Answer 2

In the World War I, Cherokee and Choctaw peoples were the ones to do the first instances of experimenting with the Native American Languages as a source for military code. So the answer to the U.S. military first experimented with Native American languages in military intelligence is in the time of the World war I.