The Scots language at that time was called "Inglis" ("Scottis" meant Gaelic, which in the written form was actually Early Modern Irish, but that is a story for another time) but that didn't mean it wasn't already recognised as being distinct. The court-dialects of the two kingdoms were similar - the whole area of Saxondom was of course divided into numerous impenetrable vernaculars - but Scots was generally considered a broadly-comprehensible foreign language by the English, certainly from the time of the Elizabethans. It was a change in the usage of the names for these languages, not the spoken language of anybody in Scotland, which brought about this crystallisation of attitudes.
It was only with the Reformation, the Book of Discipline, and the Union that people started deliberately writing in literary English as the proper language of officialdom and high culture in Scotland. And they never altogether stopped writing in various types Scots.
It's not a question of Scots
coming to be seen as seperate but of
continuing to be seen as such.
even Burns was never given anything like the appreciation that say Wordsworth or Byron was.
What does that signify, though? Scots-language literature obviously has a smaller market than English literature, because we're the smaller country. The important thing is its popularity and usage in Scotland.
Anyway, some Scots who not only wrote in English but had a profound influence on English literature also wrote in Scots.
What I really was thinking of was not so much a regional dialect as much as a stand-alone language as different from English as Dutch is from German.
There isn't any neat way of saying how alike two forms of speech actually are, but Scots certainly was as distinct from English as Dutch from German by the only really relevant criterion: people
said it was foreign.
The reality of the age before telegraphy and public schools was that people from Edinburgh had no idea what the hell people from the north-east were saying, people from London had no idea what people from Yorkshire were saying, and people along both sides of the border understood each-other readily enough.