Fixing mkv files
I have a bunch of mkv files with foreign audio, but the subtitle track doesn’t have a language set, so Plex doesn’t know it should auto-select the subtitles.
I finally found a solution: Good ol’ mkvtoolnix, or mkvpropedit to be exact.
Just run this in the folder with the offending files:
for file in $(ls *mkv); do mkvpropedit $file --edit track:s1 --set language=en; done
This sets the language of the first subtitle track to english.
See also the official docs