There are ancient inscriptions in Wales that no one has been able to read in modern times. Deciphering an unknown langauge, not related to known languages, when it is written in an unknown script is a feat of linguistics that transcends mere cryptanalysis and has, so far, rarely or never been done.
However, one cannot discount the decipherment of Egyptian hieroglyphics the Rosetta Stone. I imagine it would be extremely difficult to decipher a language that very structurally different from what is known. It is interesting to speculate about artificial grammars. Most human and computer languages (with interesting exceptions, such as the Hopi Indians' concept of time -- others can describe computer exceptions) follow the verb, noun, preposition-based method of signification. I don't know whether any work has been done on constructing a seriously structurally different artificial grammar. Jorges Luis Borges has an interesting riff on the idea. If anyone's interested, I'll dig out the details.
"Poor Man's Crypto", possibly even better than digital crypto, may consist in creating an artificial language together, and then using it whenever you don't want to be eavesdropped on.
As in thieves' cant. Or Irish. Speaking Irish was such a crime that schoolchildren wore a 'tally-stick' around their necks. Each Irish word meant a notch on the stick. A certain number of notches meant punishment, probably not gentle. Those who imposed this system were Irish, not English. In France, the Africans have an argot called verlins (an anagram of l'invers - the inverse) where syllables within words are transposed, or words are spoken backwards. Not very popular with the Corps Republican Securite. All the best Tiarnan