A purported programming language should be accepted as such if and only if it is capable of addition of natural numbers and primality testing of natural numbers.
Peter's post goes into more depth but the point is that not just anything one might want to call a programming language counts as a valid language you can answer in here on PPCG. The main exception to this rule is when the original poster of a challenge specifically allows languages that would otherwise be forbidden.
The problem I see, however, is that there may be many old answers on this site that use languages that are now considered forbidden. A user, especially a new user, could easily see these answers and assume that the language is allowed (perhaps never having seen Peter's post). Then when they answer in that language they will be disheartened by downvotes and fussy comments.
Additionally, if a user wants to answer in a relatively uncommon language that they may just be learning, it might not be clear if the language satisfies our definition of a programming language or not. For example, if someone is just trying to output
Hello, world! then their focus will not be on proving that the language can generate primes.
I suggest we make a list of supposed "programming languages" that are forbidden by default here on PPCG so all users can easily make sure their languages are valid without wasting time writing an entire answer or specifically asking on meta.
Each answer here should cover one purported programming language (or a group of highly related languages) and explain why it doesn't satisfy our criteria. This is not the place to invalidate languages you don't like. Only languages that clearly do not satisfy our definition of a programming languages should be posted.
Answers here only invalidate languages if they have a score (upvotes minus downvotes) of at least three. This ensures that no single person can invalidate a language. (I'm completely fine with changing this +3 vote threshold.)