Either single language questions are forbidden or they aren't. If they are forbidden, close this. If they are allowed, let the OP restrict the question as they like.
There are situations in which language-specific challenges make sense, such as king-of-the-hill. I'm not talking about those. For code-golf, do we have a good reason to allow them at all? We already discourage them, and arguably it falls under the things to avoid when writing challenges, since it disallows all languages but the one specified.
Should we make a rule explicitly banning code golf challenges that are specific to a single language?