[Theodores]

Imagine an article about sidewalks, there could be an explainer for Brits that explains that 'they mean the pavement' and another about 'customary units'. Americans would not need the explainers, so they could be made visible only to international readers with:

   [lang=en-us] aside { display:none; }

[crabmusket]

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.

[Theodores]

Correct!

It has been a year since I last checked, and I did use JS and 'navigator'.

However, importantly for me, I was able to avoid a trip back to the server for the few bits I wanted - introduction - in different locales.

[Theodores]

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.