http://trac.bunkus.org/ticket/1082

I was writing a chapter editor to help speed up applying chapters to all my MakeMKV Rips. I'm not up to messing around with container internals so I was using MKVExtract and MKVPropEdit to do the dirty work. I was going through the Matroska Specs, trying to follow them to the letter in creating the XML file to pass to MKVPropEdit. According to the specs, if given no other language, chapter names default to "English", so I dutifully wrote my program to remove the "eng" language tag if it was the only language designator for a chapter's name. So my program spat out the xml file and I loaded it into the MMG chapter editor to check if it was ok. MMG loaded the file, but decided to mark all the chapters as "und"... The same thing happened if I added the chapters to the file using MKVPropEdit or MKVMerge. So I stupidly e-mailed Mosu and asked where I'd got it wrong...

https://www.bunkus.org/videotools/mkvtoolnix/doc/ChangeLog

2014-09-29 Moritz Bunkus <moritz@bunkus.org>

* mkvmerge, mmg's chapter editor: fixed the default value for the
»language« element if it isn't present in a chapter XML file.
I can't even bring myself to finish the damn chapter editor, apparently no one ever bothered to wonder what happened if you combined ordered chapters with sub-chapters and I just can't operate with that hanging over my head.

If I had something better to do, I'd just quit at this point.