g++ Locale ??

[Papa <papa@xxxxxxxxxxx>]

COLLECT_GCC=C:\gnu\mingw-w64\mingw64\bin\g++.exe
COLLECT_LTO_WRAPPER=C:/gnu/mingw-w64/mingw64/bin/../libexec/gcc/x86_64-w64-mingw32/6.3.0/lto-wrapper.exe
Target: x86_64-w64-mingw32
OS: Win 8.1
class{
   private:
    std::locale current_locale;
   public:
    void setGlobalLocale(const std::string& str) {
        current_locale.global(std::locale(str)); //<< RunTimeError>>
    }
};

I'm writing a wrapper for std::locale, but right at start-up the first 
problem.
Why would this member function give me a Run Time Error?

Any help is much appreciated.

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