I was watching Outlander and almost every single Scottish character is a bilingual English and Gaelic speaker. However as I understand it before the Clearances the Highlands were still a pretty foreign and distinct culture from their neighbours down south, so maybe the English language wasn't so common back then.
I answered a variant of this question here. The short answer is that the earliest significant attempt to assess the languages spoken in Scotland dates to 1755 and suggested that almost all Scots living north a line drawn from the Clyde to the Tay were monoglot Gaelic speakers before 1745. There would have been exceptions - the English language was common in the two large northern towns, Inverness and Aberdeen, and in east coast ports, and the English presence in the Highlands ramped up considerably after the 1715 rebellion, after which a garrison of up to 20,000 was based in the region. In addition, clan chiefs, innkeepers, merchants and any other Highlanders brought into regular contact with these Englishmen, or those who had reasons to travel to or live and work on occasion in the Lowlands might have acquired reasonable English. But the average Highland farmer would rarely encounter English speakers and would speak little or no English.
Sources
John C Gibson, "The roots of Jacobitism and the Disarming Act" in Traditional Gaelic Bagpiping, 1745-1945 (1998)
Donald MacAulay, "The Scots Gaelic Language" in MacAulay [ed], The Celtic Languages (1992)