Cuilionn has it right of course, "broad" or more formally "lowland" Scots, is classified as a discrete language, derived from Old English. The other languages which developed from Old English are I believe, Kentish, Dorset, and Middle English (As used by Chaucer etc). It is definitely Germanic and nothing to do with the Celts who were consigned to undesirable parts of the country by that time.

Mind you if they had known that the celts were to wreak their revenge by inflicting Enya on us and convincing virtually everyone that all folk songs are "Irish" they might have acted differently! (grin)

Pete M