The Anglo-Saxons kingdoms included much of Scotland. Northumberland went way north of the Humber. Scots English was not English that developed in the south and then moved north. It developed contemporaneously.
While all of these facts are more or less accurate, as a result of revisionist history and the impact of a defined border more recently than that, Scotland when taken as a whole is considered a Celtic nation as opposed to Anglo-Saxon.
http://en.wikipedia.org/wiki/Celtic_nations
I will jump into this one
Scots-English is a language that developed after the Union of the Crowns. The Germanic language of Scotland was Scots which only slightly less far removed from 'English' as Frisian is removed from English today. It's home was the sweep of Scotland from the Firth of Forth (including Edinburgh) to the English border and also, somewhat independently (at last initially) in the North East. It developed from Northumbrian Old English. The area to the south west (the Kingdom of Strathclyde) spoke an early iteration of Welsh until the collapse of the kingdoms of the Britons where it then made way for both Gaelic and Scots. Gaelic was the predominant language of the north of Scotland which as late as the 1800's was spoken from a line that stretched as far south as about 20 miles north of Glasgow.
Now you have to remove today's demographics from the map. At the Act of Union in 1707 the population of Scotland was about 1,000,000. It is estimated that it stood at around 800,000 in the very early 1600's. The population of Scotland, in terms of distribution at this time was more heavily concentrated in the Highlands and islands. 'Scots' itself was technically an umbrella term for a collection of tongues that people could have a simple conversation across including Doric, Norn, Lallans etc. These had not yet been standardised and indeed never would be by the time of the Union and the permeation of Scots-English. As a language, Gaelic was at the very least the largest in terms of languages spoken by plurality if not by outright majority.
Gaelic's strength ironically was probably greater from the 1350's to 1600's than it was from 1000 to the 1350's simply as a result of a higher mortality rate as a result of the Black Death which also led to a migration northwards.
That is why Scotland, at least in terms of language should be rightly considered a 'Celtic' nation from it's foundation to the acts of union.
