Here's another thing you can do. This works - I tried it with Prison Break - 3x07 - Vanomos.
If you will view from the computer do the following:
1) Download .ssa file from
:: ThaiSubtitle.com :: ศูนย์รวมซับไตเติ้ลสำหรับคนไทย, Translate Movie Subtitles English into Thai
2) Rename the file so that it matches the name of your avi file exactly (leave the extension as 'ssa' or if you like you can change it to 'srt' - it doesn't matter)
3) Use BS Player to watch the movie
4) BS Player. While movie is playing right click on screen and select subtitles --> subtitle properties --> font. In the font menu, choose a font that supports thai script like 'Angsana New or AngsanaUPC. Make sure you have selected 'Thai' in the 'script' drop down box on that font options screen.
And that's it - very simple.
If you want to see thai subs in your standalone player you will need to convert to 'sub' files from 'ssa' or 'srt' since most standalone players don't support Thai character sets for srt files.
Note: I couldn't get VLC to play ssa files with Thai character sets - I don't know why. If I figure it out I'll let you know. It is strange - I've converted the character set in the ssa files to UTF-8 and set the text renderer in VLC for UTF-8 and they still don't show up. The subs appear as white boxes.