Select subtitle stream from the mkv container automatically

Select subtitle stream from the mkv container automatically
if it has a DEFAULT flag on it. User overrides for
preferred language should continue to take precedence.
The current versions of MKVToolnix correctly support DEFAULT
track tag but some older versions don't. If you want to have
subtitle streams in your mkv file, but not have them activate
by default, try remuxing any files causing you trouble, with
the current version of MKVToolnix, and turning the DEFAULT
track flag option to NO for all subtitles streams.
MKVToolnix is available from http://www.bunkus.org/videotools/mkvtoolnix/