There is a setup for playing mkv with subtitles ... on Android ....
What is needed:
Serviio of course
UpnPlay (availible on the market)
MoboPlayer (also available on the market) together with the MoboCodec (you are going to be prompted when starting Mobo the first time if it is required)
Setup: Basically ... nothing but in Mobo to set to ""Default using soft-decoding" in the "Playback" section and set the subtitle size. Everything else is default (assuming Serviio is working correct).
When you start UpnPlay you see servio to select with all options, when selecting a File take Mobo as player ... that's it
Note hat using Not the setting in Mobo "Default using soft-decoding" caused mobo to crash (or better to become unresponsive)