Hey there. Is there a similar task for doing this for Wikidata? mis is a reserved language name for "no code", so we can't create a language under that.

We could create qaa (or similar) for this use case.

I don't know about Wikidata. I'm forced to use mis on the Lexemes, as it's what is recommended to do for languages that don't have ISO codes (and probably won't have for the coming decades...). Ordia does a similar thing on some of its games after I requested Lorrain be added to them: it appears under "mis".
Feel free to do whatever is needed on the technical side for this to work. On the French Wiktionary, the regional languages of France that don't have ISO-codes use a "custom" ISO-code (i.e. their full language name in French: gallo for gallo, lorrain for lorrain, berrichon for berrichon...). Anything will do, as long as I can start having fun with lorrain, and help pave the way for other languages that don't have ISO-codes.

A possible option is to use the Wiktionary code for Lorrain: roa-lor.

In T420823#11742618, @ChaoticVermillion wrote:

A possible option is to use the Wiktionary code for Lorrain: roa-lor.

Potentially we could use roa-x-lor (roa-lor implies it's a standard recognised sub-language of roa, but that's just the concept of Romance languages generally; adding an 'x' shows that it's a bespoke variation). We've been asked to only use standard BCP47 codes in future for all Wikimedia things to reduce the breakages with various tools.

Does the Language Committee have a view on these things generally?

If roa-x-lor follows the standard BCP47 codes then I don't have any objections - my goal is only to be able to work with lorrain on Wikifunctions and Abstract Wikipedia :).

I don't know much about the Language Committee, and afaik they only work on the linguistic editions of the sister projects? I don't see how they would be relevant here, but I'm probably mistaken.

In T420823#11743720, @Poslovitch wrote:

I don't know much about the Language Committee, and afaik they only work on the linguistic editions of the sister projects? I don't see how they would be relevant here, but I'm probably mistaken.

The Language Committee decide whether we accept support for individual languages at Wikimedia. We in Wikimedia engineering do what they tell us around these. I've written to them, let's see what they say.

Alright, then how do I contact them for this issue?

Note currently defining language codes with -x- will cause issues. See T325728: Defined language codes containing -x- generate errors when used for lexemes

In T420823#11770114, @Bugreporter wrote:

Note currently defining language codes with -x- will cause issues. See T325728: Defined language codes containing -x- generate errors when used for lexemes

Cause issues in Wikidata, specifically.