Don't write "normal" language elements(like the ChapLanguage) if the language code is not an ISO639-2 code
When I activate the ISO639-3 and -5 codes and set the language to an ISO639-3 or -5 code I get a warning that some players can't handle such a language code.
But MTX writes additionally a "normal" language element.
For example the "alu" language code is written in the ChapLanguage element, but this is an invalid value for that element. ChapLanguage element allows only ISO639-2 codes (and combined with a dash and the country code).
I think it is better to omit the "normal" language element if the language code is not an ISO639-2 code. A player reads automatically the "eng" code but have to ignore them, because a LanguageIETF is present and must be used.