I have an IR remote with a subtitle button for my kodi box that does work (though I have no idea what command it is sending), it cycles through the available subtitles. However I would like to do this without relying on IR control.
To rule out a connection issue, try restarting Home Assistant and shortly after the restart, and issue the command then. (Vice-versa; Restart Kodi and issue the command shortly afterwards)
It’s connected fine, I have lots of other controls that work as intended (play, pause, big skip, etc…)
Before I shut down for the night I did notice that for the file I was playing the subtitles are internal, I.e. in the mkv container, rather than in an external file. I’ll do some more testing tomorrow night to see if the command only works with external subtitle files.
I’ve had a quick look through Kodis source code and the SetSubtitle function hasn’t been touched in years (neither internal nor API), so I’d assume it might have something to do with int/ext subtitles.
Maybe try a simple video with an embedded one and an external .srt
I’ve confirmed it working with 2 external .srt files.