languages should probably be as wide as possible, rather than including large numbers of mutually-intelligible regional variations. I can see why for various reasons people might want that latter info, but I don't think it'll help usability. Possibly allow tagging of variations, but search for them as the more general case.

Good point: As a feature this will be less useful if everybody claims to be writing in a subtly different language to everybody else! But, people may want these variations as an identity thing.

Maybe some sort of approach along the lines of the mood-icon dropdown, where people can select a (broad) language from a list but then customise the text that's used to describe it? That way people can display what they want but the broader category is used for searching? I'm sure there would still be awkward edge-cases, but...