Show translation from coreutils message catalog string

Actually test using the coreutils catalog, as this was broken for many
years due to using a wrong catalog name and not adding /usr/share/locale
as a search prefix, but went unnoticed.

So add a menu item to show translation of a string in this catalog to
make testing that this works simpler.

samples/internat/internat.cpp

@@ -42,6 +42,7 @@
 // coreutils package (which is always installed) as an example.
 #ifdef __LINUX__
     #define USE_COREUTILS_MO
+    static bool g_loadedCoreutilsMO = false;
 #endif // __LINUX__
 
 // ----------------------------------------------------------------------------
@@ -72,6 +73,9 @@ public:
 public:
     void OnTestLocaleAvail(wxCommandEvent& event);
     void OnAbout(wxCommandEvent& event);
+#ifdef USE_COREUTILS_MO
+    void OnCoreutilsHelp(wxCommandEvent& event);
+#endif // USE_COREUTILS_MO
     void OnQuit(wxCommandEvent& event);
 
     void OnPlay(wxCommandEvent& event);
@@ -170,6 +174,9 @@ wxCOMPILE_TIME_ASSERT( WXSIZEOF(langNames) == WXSIZEOF(langIds),
 wxBEGIN_EVENT_TABLE(MyFrame, wxFrame)
     EVT_MENU(INTERNAT_TEST, MyFrame::OnTestLocaleAvail)
     EVT_MENU(wxID_ABOUT, MyFrame::OnAbout)
+#ifdef USE_COREUTILS_MO
+    EVT_MENU(wxID_HELP, MyFrame::OnCoreutilsHelp)
+#endif // USE_COREUTILS_MO
     EVT_MENU(wxID_EXIT, MyFrame::OnQuit)
 
     EVT_MENU(INTERNAT_PLAY, MyFrame::OnPlay)
@@ -272,7 +279,8 @@ bool MyApp::OnInit()
     //
     // if it's not installed on your system, it is just silently ignored
 #ifdef USE_COREUTILS_MO
-    m_locale.AddCatalog("coreutils");
+    wxLocale::AddCatalogLookupPathPrefix("/usr/share/locale");
+    g_loadedCoreutilsMO = m_locale.AddCatalog("coreutils");
 #endif // USE_COREUTILS_MO
 
     // Create the main frame window
@@ -330,6 +338,10 @@ MyFrame::MyFrame(wxLocale& locale)
     macro_menu->Append(INTERNAT_MACRO_9, wxGETTEXT_IN_CONTEXT_PLURAL("context_2", "sing", "plur", 2));
 
     wxMenu *help_menu = new wxMenu;
+#ifdef USE_COREUTILS_MO
+    help_menu->Append(wxID_HELP, _("Show coreutils &help"));
+    help_menu->AppendSeparator();
+#endif // USE_COREUTILS_MO
     help_menu->Append(wxID_ABOUT, _("&About"));
 
     wxMenuBar *menu_bar = new wxMenuBar;
@@ -383,6 +395,29 @@ void MyFrame::OnAbout(wxCommandEvent& WXUNUSED(event))
     dlg.ShowModal();
 }
 
+#ifdef USE_COREUTILS_MO
+
+void MyFrame::OnCoreutilsHelp(wxCommandEvent& WXUNUSED(event))
+{
+    if ( g_loadedCoreutilsMO )
+    {
+        // Try showing translation of a message used by coreutils: notice that
+        // this string isn't inside _(), as its translation is supposed to be
+        // already present in the coreutils catalog, we don't need to extract
+        // it from here.
+        const char* const msg = "      --help     display this help and exit\n";
+        wxLogMessage("Translation of coreutils help option description is:\n%s",
+                     wxGetTranslation(msg));
+    }
+    else
+    {
+        wxLogMessage("Loading coreutils message catalog failed, set "
+                     "WXTRACE=i18n to get more information about it.");
+    }
+}
+
+#endif // USE_COREUTILS_MO
+
 void MyFrame::OnPlay(wxCommandEvent& WXUNUSED(event))
 {
     wxString str = wxGetTextFromUser