So lets start by installing the needed tools for extracting the subtitles from the .mkv files:

brew install mkvtoolnix

Next we want to see all tracks of the .mkv file:

mkvmerge -i my-movie.mkv

This will give you a similar output:

File 'my-movie.mkv': container: Matroska
Track ID 0: video (MPEG-4p10/AVC/h.264)
Track ID 1: audio (DTS)
Track ID 2: audio (DTS)
Track ID 3: subtitles (VobSub)
Track ID 4: subtitles (VobSub)
Chapters: 16 entries

So we see that this .mkv has one video, two audio (english and german) and two VobSub subtitles tracks (english and german). We are interested in the two last tracks and with these commands you can extract them:

mkvextract tracks my-movie.mkv 3:sub3.srt 4:sub4.srt

Currently I only extracted VobSub subtitles and this guide deals with the .sub/.idx files and convert them to the .srt format. It could also be that there are already .srt files in the .mkv files, then you do not need to convert anything and just check the language of the subtitles by opening them in any text editor and check the language.
This will result in four files named sub3.idx/sub3.sub and sub4.idx/sub4.sub in the same directory. Currently we do not see what language the subtitles are so we need to convert them to the .srt format. Before we are able to do this we need to install the needed tools:

brew install --all-languages tesseract
brew install --HEAD https://github.com/ruediger/VobSub2SRT/raw/master/packaging/vobsub2srt.rb

After that you can execute the following commands to convert the subtitles from the .idx/.sub format to the .srt format:

vobsub2srt my-movie

Please note that you just add the name of the .idx/.sub subtitle files. So you do not need to add any file extension just the name. This process can take some time depending on the size of the subtitles.