Navajo language
During World War II, the government used a code based on the Navajo language to use as secret communication. These code talkers would relay secret messages using the code. It is the only military code that hasn't been broken. The code worked by assigning Navajo words to common military phrases. The code proved for the most part effective for conveying secrecy, as the Navajo language was often not learned by spies attempting to break American codes.