Support creating Unix wxUILocale for languages without region

Unlike MSW and macOS, creating wxUILocale for e.g. "en" doesn't
necessarily succeed under Linux even if "en_US" is available, so try to
find a working language+region combination if just language doesn't
work.

This requires access to the languages database, so add a private
wxGetLanguageInfos() to avoid having to depend on wxLocale just for
this.

include/wx/private/uilocale.h

@@ -12,6 +12,12 @@
 
 #include "wx/localedefs.h"
 #include "wx/string.h"
+#include "wx/vector.h"
+
+typedef wxVector<wxLanguageInfo> wxLanguageInfos;
+
+// Return the vector of all known languages.
+const wxLanguageInfos& wxGetLanguageInfos();
 
 // Function returning hard-coded values for the "C" locale.
 wxString wxGetStdCLocaleInfo(wxLocaleInfo index, wxLocaleCategory cat);