Hi!
For instance, we not only need identifiers for German, Swiss and Austrian German. We also need identifiers for German German before and after the spelling reform of 1901, and before and ofter the spelling reform of 1996. We will also
Theoretically, BCP 47 should be able to handle this? E.g. they have sl-IT-rozaj-biske-1994 as an example. But we probably shouldn't try to construct these tags ourselves, but instead let editors specify them.
need identifiers for the "language" of mathematical notation. And for various
That's where using zxx-math or something like that would be useful? Or we could omit any tags from those at all.
The only system I know that gives us that flexibility is Wikidata. For interoperability, we should provide a standard language code (aka subtag). But a language code alone is not going to be sufficient to distinguish the different variants we will need.
I think it can be, using BCP 47 extensions, but Wikidata team should not be taking care of it - instead, Wikidata editors should do it by assigning language tag properties to specific Wikidata items.