Add wxUILocale with minimal functionality

Use it in the sample to show what (little) it can do right now.

include/wx/uilocale.h

@@ -0,0 +1,66 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/uilocale.h
+// Purpose:     wxUILocale class declaration.
+// Author:      Vadim Zeitlin
+// Created:     2021-07-31
+// Copyright:   (c) 2021 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_UILOCALE_H_
+#define _WX_UILOCALE_H_
+
+#include "wx/defs.h"
+
+#if wxUSE_INTL
+
+#include "wx/localedefs.h"
+#include "wx/string.h"
+
+class wxUILocaleImpl;
+
+// ----------------------------------------------------------------------------
+// wxUILocale allows to use the default UI locale and get information about it
+// ----------------------------------------------------------------------------
+
+class WXDLLIMPEXP_BASE wxUILocale
+{
+public:
+    // Configure the UI to use the default user locale.
+    static bool UseDefault();
+
+    // Get the object corresponding to the currently used locale.
+    static const wxUILocale& GetCurrent();
+
+    // Get the platform-dependent name of the current locale.
+    wxString GetName() const;
+
+    // Query the locale for the specified information.
+    wxString GetInfo(wxLocaleInfo index,
+                     wxLocaleCategory cat = wxLOCALE_CAT_DEFAULT) const;
+
+    // Note that this class is not supposed to be used polymorphically, hence
+    // its dtor is not virtual.
+    ~wxUILocale();
+
+private:
+    // Ctor is private, use static accessor to get objects of this class.
+    wxUILocale() : m_impl(NULL) { }
+
+    // Used by UseDefault().
+    //
+    // Note that this object takes ownership of the provided pointer and will
+    // delete it in dtor.
+    void SetImpl(wxUILocaleImpl* impl);
+
+
+    static wxUILocale ms_current;
+
+    wxUILocaleImpl* m_impl;
+
+    wxDECLARE_NO_COPY_CLASS(wxUILocale);
+};
+
+#endif // wxUSE_INTL
+
+#endif // _WX_UILOCALE_H_