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Many thanks to Bob Ippolito for finding what's going on...

It turns out that CoreFoundation.framework calls 
__setonlyClocaleconv when it is loaded. As the name suggests this 
breaks the locale API's. I don't know what whoever added this 
"functionality" was smoking, but at least he had enough sense to make it 
possible to restore functionality.

The prototype for __setonlyClocaleconv is: extern int 
__setonlyClocaleconv(int val);

Experimentation shows that setting val to a non-zero value breaks 
setlocale and setting it to 0 re-enables setlocale. The function returns the 
previous setting.

This means it is possible to create a workaround for this issue:

1) Add configure and/or weak-linking magic to detect the function
2) Call oldValue = __setonlyClocaleconv(0) before using a locale API
3) Call __setonlyClocaleconv(oldValue) after doing so.

Would such a patch be accepted?