Here's a general overview of what you're going to have to do to fix the file:
1) Rip the file to your computer (You said it was VCD, so that should be MPEG1).
2) Break apart the MPEG file and extract the video and audio portions.
3) Using an audio tool (Something better that Windows Recoder, you'll probably need Sony's Sound Forge), you'll need to seperate the left and right channels into seperate audio files. And yes, using the right tools, seperating stereo audio channels is easy. Just remember to not overwrite the original sound file.
(Optional) Duplicate the mono audio track so you end up with a dual-mono sound file (so it will come out of both speakers on your TV or the front 3 on a surround system)
4) Using a DVD/VCD creation tool, create your new VCD using the original video file you split apart and the correct language audio track.
If you don't have access to a good sound manipulation program or this is over your head, you're better off having someone do the work for you. YMMV.