It's probably reading your XP localization info that you set in the Control Panel >> Regional and Language Options and setting it based on that. If you don't want to change those settings you can try going to the VLC\locale folder and see if you have a subfolder for Thai, probably named th. Don't delete it but try renaming it to something like th.test and then start up VLC. If those are the messages that should keep VLC from being able to find them and it should revert to English. If that doesn't fix it then you can go back and rename the folder back to th.

I switched to SMPlayer a while back. It's a freebie too and includes a few more codecs - including RealPlayer format. If you're interested you can check it out at http://smplayer.sourceforge.net/.