You can remux the videostream and the wanted audiosteams with Rejig (but make sure that you enable "Copy subtitle colours" and browse therefore for the original ifo (make sure the original DVD remains on your PC in a Video-TS-folder) of the original DVD. Otherwise the subtitles may have a green colour.

You could also remux the streams with Muxman.

When finished, you should have the original (unprocessed) DVD in a Video-TS folder and the new muxed Video in another Video-TS-folder.

Then replace the files of the new Video-TS folder into the unprocessed (original-DVD) Video-TS folder as described in this guide.

You can start with step 6 of the guide. You also need Vobblanker for replacing, its freeware.