How can I watch a DVD using Mountain Lion's DVD player with multiple subtitle tracks, such as english and chinese, simultaineously?
MacBook Air (13-INCH, MID 2011), OS X Mountain Lion (10.8.3)
This will depend on the media player. I do not believe DVD Player in OS X offers this, and don't think even versatile players like VLC or MPlayer support it. Most give you the option for one subtitle track at a time.
One option is to use a transencoder like Handbrake to rip the DVD and "burn in" one of the subtitle tracks but include the others. This way the burned in one will always display, and you can then enable the second one. It's a bit of a workaround, but will ultimately do what you're looking for.
How can I watch a movie and have two subtitle tracks at once?
