How to delete orphan subtitles?

I’d like to automate cleaning up orphan files on my NAS which is serving files to KODI.

When I delete a watched tv episode in KODI, it leaves the corresponding subtitle file untouched in the directory.

I’d like to have a Bash script to recursively go through the TV folder and delete subtitle files (.srt, or sometimes .xx.srt for different language, where xx is the 2-letter language code) which do not have a corresponding .mkv, .avi or .mp4 file.

My folder structure looks like this:

After deleting episodes it looks something like this: