Merge subtitles into a Matroska container on the command line
Actually pretty darn easy – surely faster than using the GUI.
mkvmerge -o <output>.mkv --default-track 0 --language 0:<language> <subtitles>.srt <input>
You can list the language codes by invoking mkvmerge with the
--list-languages (e.g. eng for English, ger for German) parameter. The
--default-track parameter just sets the newly muxed subtitles as default for the player to use. If you are muxing multiple subtitles, you of course have to change the language code preceding number.