In light of the recent discussions about using ICU on OS X, I looked
into the Core Foundation locale functions (Core Foundation = traditional
Mac API in OS X, as opposed to the Unix/POSIX APIs).
Attached is a proof of concept patch that just about works for the
sorting aspects. (The ctype aspects aren't there yet and will crash,
but they could be done similarly.) It passes an appropriately adjusted
collate.linux.utf8 test, meaning that it does produce language-aware
sort orders that are equivalent to what glibc produces.
At the moment, this is probably just an experiment that shows where
refactoring and better abstractions might be suitable if we want to
support multiple locale libraries. If we want to pursue ICU, I think
this could be a useful third option.