English and Scots are also of course remnants of a foreign invasion.
So are the Gaelic languages. (It's turtles all the way down).
Gaelic arrived in Scotland within 100 years of English arriving in England. They are both attested to arrive in around the 4th - 5th AD century IIRC. Before that Scotland spoke Pictish (which is not known in the modern era and may/maynot have been a Brythonic language) and a language related to Welsh in the Lowlands. Gaelic is a very interesting language and should absolutely get championed and preserved, but it is not the ancient language of Scotland, and hasn't really been spoken there much longer than the English language was in the UK.
And, the situation and standing of gaelic in Ireland and Scotland are quite different. In Ireland, gaelic is strongly associated with the primary, and successful ethnonationalist movement. In Scotland, at the end of the day gaelic is a remnant of a foreign invasion, and is also historically associated with catholicism, so is often seen as the “other”. This makes it more difficult to whip up enthusiasm to learn it, even among die hard Scottish nationalists. This whole situation is quite unlike Ireland and even Wales, it would be at best a distraction in the article.