Four peoples—the Picts, the Scots, the Britons, and the Angles—were eventually to merge and thus form the kingdom of Scots.
The Picts occupied Scotland north of the Forth. Their identity has been much debated, but they possessed a distinctive culture, seen particularly in their carved symbol stones. Their original language, presumably non–Indo-European, has disappeared; some Picts probably spoke a Brythonic Celtic language. Pictish unity may have been impaired by their apparent tradition of matrilineal succession to the throne.
The Scots, from Dalriada in northern Ireland, colonized the Argyll area, probably in the late 5th century. Their continuing connection with Ireland was a source of strength to them, and Scottish and Irish Gaelic (Goidelic Celtic languages) did not become distinct from one another until the late Middle Ages. Scottish Dalriada soon extended its cultural as well as its military sway east and south, though one of its greatest kings, Aidan, was defeated by the Angles in 603 at Degsastan near the later Scottish border.
The Britons, speaking a Brythonic Celtic language, colonized Scotland from farther south, probably from the 1st century BC onward. They lost control of southeastern Scotland to the Angles in the early 7th century AD. The British heroic poem Gododdin describes a stage in this process. The British kingdom of Strathclyde in southwestern Scotland remained, with its capital at Dumbarton.
The Angles were Teutonic-speaking invaders from across the North Sea. Settling from the 5th century, they had by the early 7th century created the kingdom of Northumbria, stretching from the Humber to the Forth. A decisive check to their northward advance was administered in 685 by the Picts at the Battle of Nechtansmere in Angus.
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Up to the 11th century the unification was the work of a Gaelic-speaking dynasty, and there is place-name evidence of the penetration of Gaelic south of the Forth. But from then on, the Teutonic English speech that had come to Scotland from the kingdom of Northumbria began to attain mastery, and Gaelic began its slow retreat north and west. This is not obscured by the fact that from the 12th century onward Anglo-Norman was for a time the speech of the leaders of society in England and Scotland alike. By the later Middle Ages the language known to modern scholars as Old English had evolved into two separate languages, now called Middle English and Middle Scots, the latter with the court of the Stewart (Stuart) kings of Scots as its focus. After 1603 the increasing political and cultural assimilation of Scotland to England checked the further development of Scots as a separate language.
То есть есть только косвенные указания на то, что на гэльском говорили к югу от залива Форф (к югу от Фалкирка и Стирлинга)