xml:lang is one of those small nuggets in the XML bundle which pretty much no one uses because they are so small and haven't been advertised much, but are truly useful.
It's a simple, standard, attribute that simply says that the content within the scope of the element on which it resides is in that language (expressed as an ISO code).
Naturally, XPath is equiped with the lang('lang') function that returns true if the current node is in the requested language (and it is smart enough to know that fr_CA is fr).
Thanks to this, publishing multilingual documents in XML+XSLT is a no-brainer. I'd used it before but in a different way (mostly to filter out content before it was processed) and while I knew it to be useful I hadn't realised just how simple it is to just tag part of a structure as being in another language and not render it. While translating my résumé it occured to me that I had just used
xml:lang without even thinking about it, and that it had just worked