As I wrote in this essay, working class Scottish people of my generation grew up writing in English but speaking in Scots. That's probably still mostly true of younger people, but I notice that they have more of a tendency to write in Scots, at least in informal communications. I'm sure this is related to the strengthening of Scottish identity, and having our own parliament, in recent years. I wonder if the number of shops selling kilts in central Glasgow is also related.
I noticed a few weeks ago that, for the first time in decades, I'm thinking in Scots, not English. I don't know how long I had lived in the US before I began thinking, and speaking (though with a Glasgow accent, which never softened), in American English, but it's been my default language for most of the last two decades. Having been back in Glasgow for five months, I now have to consciously speak English if it's required. Otherwise, my speaking voice is the same as my thinking. My default written language is still English, as it always has been.