1. mkvmerge accepts BCP 47/RFC 5646 language tags in chapter-related
command line arguments that accept languages (which had only
accepted ISO 639-2 language codes before).
2. When creating chapter atoms (e.g. when reading the simple chapter
format or when the user requests it via the `--generate-chapters`
command line option) mkvmerge will always write "ChapLanguageIETF"
elements. If the given language tag contains a valid ISO 639-2
language code, it will be written as the legacy "ChapLanguage"
element; otherwise "und" (for "undetermined") is written.
Part of the implementation of #2419.
