title_body: Has any nation ever employed a constructed language in the military as code talkers?
upvoted_answer: The Israeli military uses a relexification of Hebrew called "NADBAR", according to Conlang Wikibooks†. A relexification of a language remains the grammar (by definition) and writing system (in practice) but gains new words. Your first (semi-)conlang was probably a relexification of English or another language.
  
  Many languages used in the military are not immediately conlangs; rather, they fall into the dome of cryptology and cryptography. Quote:
  
  
    Some use the terms cryptography and cryptology interchangeably in English, while others (including US military practice generally) use cryptography to refer specifically to the use and practice of cryptographic techniques and cryptology to refer to the combined study of cryptography and cryptanalysis.
  
  
  The military also uses (forms of) signed languages. Here are some common signals:
  
  Common army signals (link instead of image, for image is too big and distracting)
  
  
  
  †Note: (from comment by b a):  
  
  
    According to Hebrew Wikipedia, "Contrary to the popular notion, the goal of Nadbar is not concealing information" (so not really cryptography) See the corresponding English Wikipedia page for its use in the US military.