Changing Matroska Properties
Matroska files can have a buttload of internal flags and options, and editing them is actually quite simple.
General
For all commands here you need to know which track number you want to work on. That track number is n.
# mkvinfo file.mkv
Disabling a Track
This is useful if you have a file with more than one audio track, and the player picks the wrong audio track.
# mkvpropedit file.mkv -e track:n -s flag-enabled=0
Changing the Default Flag
This is useful if you want to turn on a certain subtitle language by default, or you want to change the default language of a file.
n is the number of the track you want to set as default, o is the number of another track of the same type (audio or subtitle) whose default flag you want to clear. If there is more than one other default track you need to repeat the last section (starting from the -e flag).
# mkvpropedit file.mkv -e track:n -s flag-default=1 -e track:o -s flag-default=0 […]
Setting a Language
# mkvpropedit file.mkv -e track:n -s language=en