Handle wxLANGUAGE_DEFAULT in wxLocale specially under MSW too

Even if we do know about the system language, it's still better to let
OS/CRT handle it, as we may not know enough about it. E.g. "system
language" may actually be a mix of the different languages and formats
and we don't handle this case at all, while OS/CRT do just fine.

src/common/intl.cpp

@@ -547,10 +547,14 @@ bool wxLocale::Init(int lang, int flags)
 
 #elif defined(__WIN32__)
     const char *retloc;
-    if ( !info )
+    if ( lang == wxLANGUAGE_DEFAULT )
     {
-        // We're using the system language, but we don't know what it is, so
-        // just use setlocale() to deal with it.
+        ::SetThreadLocale(LOCALE_USER_DEFAULT);
+        wxMSWSetThreadUILanguage(LANG_USER_DEFAULT);
+
+        // We're using the system language, so let setlocale() deal with it: it
+        // does it better than we do and we might not even know about this
+        // language in the first place.
         retloc = wxSetlocale(LC_ALL, "");
     }
     else if ( info->WinLang == 0 )