just right click on the movie, goto audio and select the audio (language) u wanted.
that is, provided the video has more than 1 audio language.
but if it's not, then cannot change lah.

edit: so far i only know 1 video format that can has more than 1 language, and that is matroska format (.MKV).

there are others, maybe..it's just i don't know about them..

*cough*
the file must be encoded with matroska format (.MKV) in the first hand.
for example u have a video called myVideo.mpg.

then u have 2 audio files for the video, let say Audio1.mp3 & Audio2.mp3

then u encode all 3 files (video, udio1 & audio2) to matroska format.

only then u have a valid matroska files and those 2 audio can be change la..
simply renaming the files (e.g: .AVI to .MKV) will not change anything.