Ok, now that this point is clear :D
All you have to do is to make a button with Action = 72 (Cycle subtitles).
To follow the Play button example, your Cycle subtitles button will be:
Code:
[Buttons]
...
Btn3=CSubt,129,116,0,72;$lang.Actions.73
...
where CSubtN.bmp CSubtU.bmp CSubtD.bmp are the subtitle buttons images
