Well under the reign of Malcom Canmore, the Scottish Nobility and clergy tended to use Gaelic. However with his marriage to St. Margaret the English langauage was introduced as the language of the high nobility. As for the common people, what they spoke is anyone guess. There was real hodgepodge of langauges around about that time, certainly north of the Forth and Clyde Gaelic dominated while southwards you have Norsemen, Brittons, Angles and no doubt a few Celts mixed in.
The supression of lowlands Scots as a language happened around about the act of Union. Two factors tended to this, first was the fact that English became the language of power and thus it became a mark of status to use it. In fact Burns himself felt the English was a much better langauge for poetry and personally thought his best poems were written in English, although ironically he is remembered for his poems in Scots. The other was social, around about the act of union the Lowland clearances began. The lowland equivalent of the Crofter (the Cotter) was steadly forced of the land. This removed the social base for the langauge and allowed English to dominate.
The supression of lowlands Scots as a language happened around about the act of Union. Two factors tended to this, first was the fact that English became the language of power and thus it became a mark of status to use it. In fact Burns himself felt the English was a much better langauge for poetry and personally thought his best poems were written in English, although ironically he is remembered for his poems in Scots. The other was social, around about the act of union the Lowland clearances began. The lowland equivalent of the Crofter (the Cotter) was steadly forced of the land. This removed the social base for the langauge and allowed English to dominate.