Thanks Leszek; ?uselang=xx is basically what I was looking for, and
works equally well when logged-in, which is helpful. (It
doesn't work through links, but that is not a show-stopper.)
The reason this is not trivial is two-fold: 1) caching
and b) the
semantics of URLs. Serving different content from the same URL is
generally a bad thing.
Agreed. (But I prefer the heading to the way Google used to do it for
years, which was decide the UI language based on your IP address, and
ignore the headers - perhaps that was caching-related, too.)
A soltuion for this is dicussed in
<https://phabricator.wikimedia.org/T114662>, but work on this is
currently not resourced.
Thanks.
Darren