Good question. I hate smart remote controls because of the current case..
You may need to find the menu button for the list or press and hold the OK button until you get the choice. The button should be called SUB.
Not at home right now, but im pretty sure when watching a channel, press the four colored button, and choose red to change audio, or green to change subtitle. What yellow did i cant remember right now, but blue puts a digital clock in upper left corner.