According to Wikipedia, the use of Gaelic was in decline in mainland Scotland from the 15th century onwards.:
“The Gaelic language was introduced to Scotland by settlers from Ireland, probably in the 4th century.[13][14]
Scottish Gaelic itself developed after the 12th century, along with the other modern Goidelic languages. Scottish Gaelic and its predecessors became the language of the majority of Scotland after it replaced Cumbric, Pictish and in considerable areas Old English.[15] There is no definitive date indicating how long Gaelic has been spoken in today's Scotland, though it has been proposed that it was spoken in its ancient form in Argyll before the Roman period.[16] No consensus has been reached on this question; however, the consolidation of the kingdom of Dál Riata around the 4th century, linking the ancient province of Ulster in the north of Ireland and western Scotland, accelerated the expansion of the language, as did the success of the Gaelic-speaking church establishment, started by St Columba, and place-name evidence shows that Gaelic was spoken in the Rhinns of Galloway by the 5th or 6th century.[citation needed] The language was maintained by the trade empire of the Lordship of the Isles, which continued to control parts of Ulster until the 16th century.
[edit] From the Middle Ages to the end of Classical Gaelic education
The Gaelic language eventually displaced Pictish north of the River Forth, and until the late 15th century was known in Scots (then known as Inglis) as Scottis, and in England as Scottish. Gaelic began to decline in parts of mainland Scotland from the beginning of the 15th century, accompanying its decline in status as a national language, and eventually the highland-lowland line began to emerge.[citation needed]
From around the early 16th century, Scots language speakers gave the Gaelic language the name Erse (meaning Irish in Scots), and thereafter it was invariably the collection of Middle English dialects spoken within the Kingdom of Scotland, that they referred to as Scottis (see Scots language). This in itself was ironic, as it was at this time that Gaelic was developing its distinct and characteristic Scottish forms of the modern period.[17]
Scottish Gaelic was called "Erse" partly because educated Gaelic speakers in Ireland and Scotland all used the literary dialect (sometimes called Classical Gaelic) so that there was little or no difference in usage. When Classical Gaelic stopped being used in schools in both countries, colloquial usage began to predominate, and the languages diverged”.
It’s perhaps worth adding that many people from Dumfries and surrounding counties came to Ireland in the 1600s, either as a part of the Montgomery Hamilton settlement or the Plantation of Ulster. Their day to day language was English, so I think it’s pretty safe to say that English was the norm for everyday conversation in Dumfries in the 1600s anyway.
