If you have webOS 3.0 TV, you can select audio track and turn subtitles on/off using red/green buttons.
On webOS 2.0 it's only possible to turn subtitles on/off.
For older LG Smart TVs, the audio track can only be "forced" in the playlist (for streams only, not VOD!). Otherwise, stream audio language is automatically selected based on the Audio Language Settings (1st Audio) on your LG Smart TV (see picture below).
If you have webOS TV, you will need to set webOS Video Mode to NC MPEG or NC Auto in App's Settings for this "forced" function to work. This function only works for streams that it supports (usually HTTP only, not m3u8).