Hoi,
There are many valid possibilities to describe something that is not a language and language used may represent a language that does not have a language code. There is a standard for indicating languages; it allows for something like "US-American Spanish" by combining a country and a language code. This is well known.
The problem with everything that has not been recognised / standardised / defined as a language is that it is highly political. The practical side is that we can use an x in a code to indicate a special use. However, then calling it a language is problematic because a language ought to mean that its understanding is mutually exclusive.
Calling it a language code and use "expressed in" would imho work for any form of language. When Wiktionaries content is imported in Wikidata, we first have to have these languages codes agreed on. To first import the bulk is no problem. It puts pressure on the resolution of such issues and that is not half bad.
Thanks,
GerardM