title_body: "Turing-completeness" in conlangs
upvoted_answer: The best parallel for Turing completeness in human languages is the Natural Semantic Metalanguage. The NSM proposes that there is a limited set of basic semantic concepts which all human languages have, and which are not reducible to other concepts. They call these "semantic primes", because you combine them to get all other meanings, and because they can't be broken down. The list of primes, currently numbering 65, is a work in progress as new primes are proposed and evaluated, but after over 45 years of work on the theory the list has proven to be very reliable and new primes are only rarely proposed.
  
  The NSM is a good guide therefore for judging whether a conlang is "complete" - whether it would be as capable for expressing human thought as any natural language. If a conlang has vocabulary items (they can be words, affixes, or phrases) for each of the semantic primes then I would judge it to be complete.