Great, we do the same thing in Mapnik to avoid locale locks (use karma). Although, the severity of the locale mutex differs significantly on the standard C++ library implementation. For example on OS X see the difference between in test 8 between libstdc++ and libc++: https://github.com/mapnik/mapnik/wiki/Benchmark-Notes (which is the `test3` struct test at https://github.com/mapnik/mapnik/blob/master/benchmark/run.cpp#L193-L217)