If you want to use the language for labels etc, than it's not about using it as a monolingual language code.

@Mbch331 could you please elaborate?

To clarify and to re-take the example of https://www.wikidata.org/wiki/Q120043 , I want to put both the label and the P1843 in Guianan Creole (at least for the second, don't I need a monolingual language code?). Or should I keep using the 'mis' lang code with a lang qualifier?

@VIGNERON All codes accepted by ULS are automatically also accepted as a monolingual code. If you want to use it in labels it has to be a language that's part of ULS.
If you only want to use it in properties of the type "monolingual text", then it needs to be a separate monolingual code, but than it's not going to be part of ULS. T144272 is only a parent task when you need a monolingual code that's not going to be part of ULS.

This got added by myself in 887745d4db6f as per T186635: Add Guianan Creole (gcr) to Names.php.