How to display subtitles as default in an MKV file I have an MKV file with multiple subtitles.  I would like to mark one set of those subtitles as default, so that it is turned on by default when the video starts playing.  NOTE: I do not want to "burn" this subtitles set into the video (I want to be able to turn these subtitles off at any point in time if I so choose).

Is there an MKV editing tool that would allow me to do this?
There are two aspects to your query - 1)setting one of the subtitle streams as the default and 2)automatically displaying it during playback.I'm not aware of a method to force a player to display subtitles if that facility has been turned off in its settings.

For the first facility, try mkvpropedit from MKVToolNix to set the default flag for a subtitle stream. See 2nd example.