Explicitly disallow wxLANGUAGE_UNKNOWN in wxLocale::Init()

This value doesn't make here, so be upfront about it.

interface/wx/intl.h

@@ -459,9 +459,11 @@ public:
         try to translate the messages using the message catalogs for this locale.
 
         @param language
-            ::wxLanguage identifier of the locale.
-            @c wxLANGUAGE_DEFAULT has special meaning -- wxLocale will use system's
-            default language (see GetSystemLanguage()).
+            ::wxLanguage identifier of the locale. It can be either some
+            concrete language, e.g. @c wxLANGUAGE_ESPERANTO, or a special value
+            @c wxLANGUAGE_DEFAULT which means that wxLocale should use system's
+            default language (see GetSystemLanguage()). Notice that the value
+            @c wxLANGUAGE_UNKNOWN is not allowed here.
         @param flags
             Combination of the following:
             - wxLOCALE_LOAD_DEFAULT: Load the message catalog for the given locale