Assertion failure on Windows 10 if the default region/language setting is "mixed" (e.g. English / Germany)

# Summary

The following program aborts with an assertion failure on Windows 10 if the default language/region is set to `English/Germany`:

```cpp
#include

int main()
{
auto regex = boost::regex{""};
}
```

Assertion:

```
Assertion failed: r != 0, file libs\regex\build\..\src\w32_regex_traits.cpp, line 125
```

The assertion does not happen if the language/region is set to `English/United States`.

Also see [this StackOverflow thread](https://stackoverflow.com/questions/62722948/c-assertion-crash-when-using-boost-regex-lib) for another example of someone encountering this issue.

# Details

## Boost version

Boost version: 1.74.0, as downloaded from [sourceforge](https://sourceforge.net/projects/boost/files/boost-binaries/1.74.0/boost_1_74_0-bin-msvc-all-32-64.7z/download)
Binaries: `boost_regex-vc142-mt-gd-x64-1_74.lib`/`.dll` from `lib64-msvc-14.2` directory

## Operating system

Edition: Windows 10 Pro for Workstations
System type: 64-bit operating system
Version: 1809
OS build: 17763.1397

## Compiler & flags

Edition: Microsoft Visual Studio Professional 2019
Version: 16.7.3

Compiler flags/project settings set to the default for a newly created blank C++ project in VS 2019, aside from adding
the `BOOST_ALL_NO_LIB` define and adding the necessary include path and `lib` file to compile the code.

The resulting compiler command line arguments are:

```
/JMC /permissive- /GS /W3 /Zc:wchar_t /ZI /Gm- /Od /sdl /Fd"x64\Debug\vc142.pdb" /Zc:inline /fp:precise /D "_DEBUG" /D "_CONSOLE" /D "_UNICODE" /D "UNICODE" /errorReport:prompt /WX- /Zc:forScope /RTC1 /Gd /MDd /FC /Fa"x64\Debug\" /EHsc /nologo /Fo"x64\Debug\" /Fp"x64\Debug\LocaleTest.pch" /diagnostics:column
```

Linker command line arguments:

```
/OUT:"C:\Users\nwu\source\repos\LocaleTest\x64\Debug\LocaleTest.exe" /MANIFEST /NXCOMPAT /PDB:"C:\Users\nwu\source\repos\LocaleTest\x64\Debug\LocaleTest.pdb" /DYNAMICBASE "C:\Dev\boost_1_74_0-bin-msvc-all-32-64\boost_1_74_0\lib64-msvc-14.2\boost_regex-vc142-mt-gd-x64-1_74.lib" "kernel32.lib" "user32.lib" "gdi32.lib" "winspool.lib" "comdlg32.lib" "advapi32.lib" "shell32.lib" "ole32.lib" "oleaut32.lib" "uuid.lib" "odbc32.lib" "odbccp32.lib" /DEBUG /MACHINE:X64 /INCREMENTAL /PGD:"C:\Users\nwu\source\repos\LocaleTest\x64\Debug\LocaleTest.pgd" /SUBSYSTEM:CONSOLE /MANIFESTUAC:"level='asInvoker' uiAccess='false'" /ManifestFile:"x64\Debug\LocaleTest.exe.intermediate.manifest" /ERRORREPORT:PROMPT /NOLOGO /TLBID:1
```

## Language settings

### Problematic settings (assertion triggered)


### Working settings (no assertion)