But lang in this case would be a property of the document itself, not the user agent, right?
I can't see a way in CSS to detect the user agent preferred language, but you could do this in JS and add another attribute to the document or whatever.
Actually, I looked into it some more, and SVG Switch, which should do stuff based on browser locale.
However, it was showing me 'en-US' not 'en-GB'. There is a bug in SVG switch that means it can do languages automagically but not variants. This I can work with, but it is still something unexpected and unlikely to be fixed because nobody cares about SVG.
I can't see a way in CSS to detect the user agent preferred language, but you could do this in JS and add another attribute to the document or whatever.