Native American Encryption?!

[Arthur Chandler]

  I remember hearing many years ago that one of the branches of the armed
aervices during WWII used a native American -- Cherokee? Apache? --
language to communicate. The Japanese (the story goes) went nuts trying to
crack the code -- unsuccessfully.
  Two questions:
  1) Can anyone give me a reference for this story (assuming it's true)?
  2) If one used a natural language for encryption, and the would-be code
crackers did not know it was natural language (say, Hittite), could they crack
it? I seem to remember that hieroglyphs were undecipherable until the
Rosetta Stone was discovered. But maybe current techniques would do a
better job....?