Navajo (Navaho) Indians. Referred to as Code Talkers. I believe you'll find something in David Kahns "The Code Breakers". They were used in tactical situations. To break an otherwise unknown language would require context, such as good traffic flow analysis and otherwise good intelligence. I beleive there was a program on A&E (TV) on the Navajo code talkers in the Pacific (WWII). Navajo code talkers used lots of colloquilisms and all new each other more or less. You could have ended up attacking language code in sets between two code talkers.