This is not so simple a thing as it may seem, unfortunately. The latest PowerDVD Ultra(currently the only real solution to play HD DVD and BD movies) has removed the ability to play HD DVD and BD from folders off the hard drive. So, the current advice is to create ISO images of your discs and mount them using Daemon Tools. VLC does not have the support to play it. Nero Showtime with the Blu-ray plugin apparently works. And there’s a new player that’s coming soon that’ll work, as well. But, so far, PowerDVD Ultra seems to be our best option which REALLY sucks.
To answer the OP’s question, or not as the case may be, I’ve read about subtitle issues with HD DVD movies from some studios. I believe you can mess around with the playback files in the My Documents\AnyDVDHD directory. There’s an XPL file in there. Editing this file should allow you to turn off the subtitles for a particular title. I have zero experience in doing this yet, but, it’s just an XML file. If you screw it up, just delete it and reopen the image to recreate it.