At this time, and in particular this area of The Islands and The Highlands, just before Culloden and the 1872 Eduction Act, you would be hard-pressed to find a lot of people in these areas speaking Lowland Scots--it would have been strictly Gàidhlig. Lowland Scots [their form of English] would have been the exception at this time, not the rule. But of course you couldn't make a show with nothing but Gàidhlig dialogue. It wasn't saved just for ceromonies, or politics, or because you didn't want the kids to know what you're talking about -- it was the language of the majority there at the time. This book and show is historical fiction afterall. Based on actual historical events, to be sure, but fiction nonetheless.
And it's how the language is being kept alive and taught today -- it's called "total immersion". :) The best way to learn it -- hard...but effective - lol!