I recommend you to fetch the subtitles manually and put them beside your video files.
This can happen if:
- the subtitle file is not accessible (usually on Linux systems, check the file permissions) https://support.plex.tv/articles/200288596-linux-permissions-guide/
- the ‘Local Media Assets’ agent has been disabled https://support.plex.tv/articles/200471133-adding-local-subtitles-to-your-media/
- with tv shows, when the necessary folder structure is not present (particularly the ‘Season’ subfolder) https://support.plex.tv/articles/200471133-adding-local-subtitles-to-your-media/
Plex cannot do this. You will need to use 3rd-party software. Set Audio Language tag MP4/MKV
I cannot tell you how to make SubZero work. Please ask in its release thread [REL] Sub-Zero 2.6.5.3268 - Subtitles done right
