[acjohnson55]

That point exists, but it's contextual. You could draw the line at shared identity. Or political boundary. Or the existence of a body of literature. Or a certain bar of recognition. Or a certain number of university programs. Or some concept mutual intelligibility. Or whatever. But these distinctions will conflict with one another, so there is no clear line.

I have no real authority, but to me, I see no reason why Scots shouldn't be a language. English and Scots would certainly not be the most similar languages to be considered distinct. Plenty of other varieties of the English continuum, for that matter.