Replying to my own post, in case someone stumbles on this while searching: ICU. It's rather baroque, but seems to do the right thing. I recommend writing a C++1

Replying to my own post, in case someone stumbles on this while searching: ICU. It's rather baroque, but seems to do the right thing. I recommend writing a C++11 wrapper for it (I might actually release this at some point in the future).