Move platform-specific parts of wxLocale::Init() to wxUILocale

This is tidier than using #ifdefs in the same common file and also
ensures that initializing wxLocale affects wxUILocale too, which will be
important for compatibility when the code elsewhere is modified to use
wxUILocale::GetInfo() instead of wxLocale::GetInfo() in the upcoming
commits.

This commit is best viewed with --color-moved git option.

include/wx/msw/private/uilocale.h

@@ -0,0 +1,28 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/msw/private/uilocale.h
+// Purpose:     MSW-specific locale-related helpers
+// Author:      Vadim Zeitlin
+// Created:     2021-08-14
+// Copyright:   (c) 2021 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_MSW_PRIVATE_UILOCALE_H_
+#define _WX_MSW_PRIVATE_UILOCALE_H_
+
+#include "wx/msw/private.h"     // Include <windows.h> to get LCID.
+
+#ifndef LOCALE_SNAME
+#define LOCALE_SNAME 0x5c
+#endif
+#ifndef LOCALE_CUSTOM_UI_DEFAULT
+#define LOCALE_CUSTOM_UI_DEFAULT 0x1400
+#endif
+
+// Use the specific LCID for the current thread.
+void wxUseLCID(LCID lcid);
+
+// This function is defined in src/common/intl.cpp
+wxString wxGetInfoFromLCID(LCID lcid, wxLocaleInfo index, wxLocaleCategory cat);
+
+#endif // _WX_MSW_PRIVATE_UILOCALE_H_

include/wx/private/uilocale.h

@@ -33,6 +33,15 @@ public:
     // It may return NULL in case of failure.
     static wxUILocaleImpl* CreateUserDefault();
 
+    // This function exists only for wxLocale compatibility and sets the locale
+    // corresponding to the given language.
+    //
+    // The language passed to this function is a valid language, i.e. neither
+    // wxLANGUAGE_UNKNOWN nor wxLANGUAGE_DEFAULT.
+    //
+    // It may return NULL in case of failure.
+    static wxUILocaleImpl* CreateForLanguage(const wxLanguageInfo& info);
+
     // Functions corresponding to wxUILocale ones.
     virtual wxString GetName() const = 0;
     virtual wxString GetInfo(wxLocaleInfo index, wxLocaleCategory cat) const = 0;

include/wx/uilocale.h

@@ -29,6 +29,12 @@ public:
     // Configure the UI to use the default user locale.
     static bool UseDefault();
 
+    // Use the locale corresponding to the given language.
+    //
+    // This is a compatibility function used by wxWidgets itself, don't use it
+    // in the new code.
+    static bool UseLanguage(const wxLanguageInfo& info);
+
     // Get the object corresponding to the currently used locale.
     static const wxUILocale& GetCurrent();
 

include/wx/unix/private/uilocale.h

@@ -0,0 +1,33 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/unix/private/uilocale.h
+// Purpose:     Various locale-related helpers used under Unix systems only
+// Author:      Vadim Zeitlin
+// Created:     2021-08-14 (extracted from src/common/intl.cpp)
+// Copyright:   (c) 2021 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_UNIX_PRIVATE_UILOCALE_H_
+#define _WX_UNIX_PRIVATE_UILOCALE_H_
+
+#include "wx/string.h"
+
+// get just the language part ("en" in "en_GB")
+inline wxString ExtractLang(const wxString& langFull)
+{
+    return langFull.BeforeFirst('_');
+}
+
+// get everything else (including the leading '_')
+inline wxString ExtractNotLang(const wxString& langFull)
+{
+    size_t pos = langFull.find('_');
+    if ( pos != wxString::npos )
+        return langFull.substr(pos);
+    else
+        return wxString();
+}
+
+const char *wxSetlocaleTryAll(int c, const wxString& lc);
+
+#endif // _WX_UNIX_PRIVATE_UILOCALE_H_