Hi, I will be happy to answer this to the best of my abilities. Firstly there have been many languages in what's now Scotland throughout its history. The language of Scotland historically speaking was Gaidhlig a early form of the Irish language or now refereed to as part of the Gaelic branch of languages today by our modern terminology. The romans called all the Irish Scotti and those living in Northern Britian Picts/ Pict-land or Albania from alban. Scott originally meant Irish. Pictish langauge became extinct and was replaced by Irish, pictish was probably related to Welsh, perhaps cumberic is the decedent of the pictish language or is related to it? You can still find place names of this British celtic variety in scotland even in Argyle birthplace of Scotland. Think of map instead of the Irish mac/mc. By the 12th century the territory of the lowlands of scotland had a English-Anglo Normans immigrating there in what was called Burghs replacing the Erse language the last native speaker of Gaidhlig in the lowlands was Donald III of Scotland in the 12th century. However, one could argue Gaidhlig was never spoken in any significance in the lowlands. Thus you had a division between the highlanders who mostly spoke a Irish language or was perceived to be, called Erse by the lowland Anglo-scots. By the time of the scottish war of independence the lowland people had been integrated into Scotland many things we think of as scottish are from Northumbria in England. Although, Scottish they were ethnically and culturally different to the wild Irish (Erse) or highlanders. The famous figures of scottish independence Robert the bruce, William Wallace where from the lowlands and of Anglo ancestry, although robert had some Irish and Scottish blood as his great something grandmother was Aoife MacMurrough; anyway off topic. They did romanticize Erse, although, I doubt they could speak much of the language instead speaking the language/dialect of scots. So scots was the language of the kings in Scotland, the language of Robert Burns. Malcolm III made the language of the scottish courts English. One could debate that scots is not a English language, which would make this question much easier to answer.

So now that we know how we got to this point who spoke what? Lets compare the downfall of the Gaidhlig language in Scotland to that of the situation in Ireland where the Gaelic language was suppressed and began to decline because of British colonialism starting in 1501 with the Irish plantation colonies of Elizabeth I of England, and by 1830s was a minority language thanks to a mass emigration/ eviction of millions of Irish predominately to the U.S.A during the Irish famine. The Irish famine was caused by Laissez-faire economics in England who ruled Ireland for centuries at that point, English and Scottish landlords were particularly to blame who made it impossible for the average poor tenant farmer to survive on anything but potatoes while massive amounts of good produce in Ireland was exported to British markets. When the blight hit the potato crop the British governments response was to do very little to nothing. You also had scots speakers in the north of Ireland a result of the ulster plantations a form colonialism and ethnic cleansing by the Scottish Stuarts king james and paid for by England and Scotland, who moved thousands of scots to the area of ulster to keep it loyal to the crown essentially trying to replace the Irish population in 1690, after the nine years war. King james the 1st didn't want a repeat of what queen Elizabeth had to deal with the red hughs, this was the main reason for the ulster plantations, although, it also served as a wedge separating the highlanders from the Irish. The situation in Scotland is a wee bit different, but has similarities. it had alot to do with policies in Scotland itself where English became the predominate language due to the textile industry, another reason might of been the act of union in 1707 with king james the 1st who declared English to be the official language of Scotland, but most highlanders still spoke Gaidhlig until the mid 19th century. The highland clearances are often giving as the reason for language shift. The Scottish government in the late 18th to 19th century essentially moved the already depopulated highlanders off their land to make room for sheep grazing. Thus all the Gaidhlig speakers were being evicted and forced to emigrate in the 1860s. Also the land in the highlands is very poor and so there was never many people there, life in the highlands consisted of cattle raiding between tribes.

summary: The lowlands of Scotland spoke British celtic and Gaidhlig, the Normans and anglo-saxons emigrated to the area and Scots/English became the majority of the language there, Malcolm III of Scotland made it the official language of the Scottish royalty and king James the 1st made English the official language of Scotland. The highlands, which was already fairly depopulated due to incredibly poor land and lack of natural resources remained Gaidhlig speaking until the 19th century when the highland clearances forced many native Gaidhlig speaking scots to emigrate mostly to Canada.