How to display subtitles as default in an MKV file
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.