title_body: Has any nation ever employed a constructed language in the military as code talkers? Esperanto was once used in the US army to realistically simulate the language situation when training.
  Has any nation ever employed other constructed languages in the military as code talkers?
  
  From Wikipedia article:
  
  
    Code talkers are people in the 20th century who used obscure languages as a means of secret communication during wartime. The term is now usually associated with the United States service members during the world wars who used their knowledge of Native American languages as a basis to transmit coded messages. In particular, there were approximately 400–500 Native Americans in the United States Marine Corps whose primary job was the transmission of secret tactical messages. Code talkers transmitted these messages over military telephone or radio communications nets using formal or informally developed codes built upon their native languages. Their service improved the speed of encryption of communications at both ends in front line operations during World War II.
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.