Each QuickTime movie file can have any number of tracks within the file. When the file is played, the application software explicitly "tells" QuickTime which tracks to play. For example, you could have a movie file with one video track and one audio track for English narration, one audio track for Spanish, another for German, and so on.
On playback, the application could check to see what language the System is in, and play only the appropriate audio track. The application could also play NO audio tracks if desired. This functionality is an application feature; QuickTime just offers the support for achieving it.