MKVToolnix VS OpenSubtitles.org

Fourth, try remuxing one of the problem files. Use the Multiplexer section of MKVToolNix. This will copy the contents to a new MKV file. It is like putting a letter in a new envelope when the original is damaged, but the pages inside are OK. Source: 6 months ago

If the audio tracks are displayed as unknown, the language flag needs to be set. Use tools such as MKVToolNix Header Editor to configure the language for audio & subtitle tracks. Source: 10 months ago

As for extracting tracks from MKVs, the low-level way would be to use ffmpeg directly (something like ffmpeg -i video.mkv -map 0:s:0 subtitle.sup would extract the first subtitle stream to a .sup file, the extension used for standalone PGS subtitles), but something like MKVToolNix can probably do it as well. That won't help you too much on its own though, since you now just have an external image-based subtitle... Source: 11 months ago

You could use a tool outside of Plex (like MKVToolNix) to combine the video+audio of one version with the audio+subtitles of the other. It gets trickier, or at least more tedious, if the videos aren't exactly the same, since you'd then have to account for any audio/subtitle shifting. Source: 11 months ago

Option 3: Change to MKV container. Use MKVToolNix or similar tools and remux to a MKV container. Note that this may cause problems with Dolby Vision. LG TVs must have Dolby Vision in a MP4 container, otherwise the video will not play correctly. Probably affects other Plex clients as well. Source: 11 months ago

Opensubtitles is another good option, for many languages. Source: about 1 year ago