To change the subtitle language on your LG Smart TV, press AD/SUBT on your remote. Select the language you want, then press OK. Press AD/SUBT and Back to hide the window.
To change the audio language for a channel, if you own a webOS 3.0 TV, you can choose the audio track and set subtitles to 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 "pushed" in the playlist (for streams only, not VOD!). Otherwise, stream audio language is set automatically based on the Audio Language Settings (1st Audio) on your LG Smart TV.