MKVToonNix does not write the old "Language" element of the chapter, only the new "ChapLanguageIETF" element.
Hello. (Please forgive the automatically translated text.)
MKVToolnix GUI writes only the
ChapLanguageIETF element, not the old
Language element when adding chapter using OGM text format.
Generating chapters options such as "One chapter for each appended file" will write both old and new language elements.
When a chapter is generated using a Blu-ray playlist (0000x.mpls), no chapter language element is written.