While I can't say I've worked on either this particular handset or with chinese/thai fontsets, there is a problem with a large number of Samsung handsets displaying any characters that don't fit in the 'base' 128 ASCII character set (edit: when trying to use the SIZE_SMALL font size). What I've seen when working on localizations to latin-based languages is that most accented characters just don't get displayed at all.
The way that I've seen this worked around is by ensuring that you are using the Medium-size font (ie: use SIZE_MEDIUM or SIZE_LARGE when defining the font). Otherwise I've had to use a custom font engine to do my displaying.
G'luck, and be sure to let us know what you find out.