On the default loopholes there is an answer that forbids making languages that are made specifically for a challenge. This is definitely a good idea. However, there are loopholes even to this. A user could create programming languages designed for a tag, trivializing many challenges. I believe that it is time to somehow create some concrete rules on what programming languages can be used. I believe a simple list would be overly restrictive to languages that are legitimate but not well known. These rules might include certain features that the language must include or that the language must be compatible with x. This should clear up a long standing problem I have had with answering code-golf questions.