How to show subtitles in Kodi?

I have an RPi2B running KODI via OSMC.
I am using a spare Motorola remote control via a FLIRC interface to move around in the GUI.
Now I have bought a DVD box with old TV shows and converted the disk to an ISO file, which I can play in Kodi. This works fine.
However, I would like to activate the subtitles that are present on the disk too and I do not know how in Kodi to do this via the remote control.
Any ideas what can be done?