I think of buying this N800 unit but the problem is most of website I often surf is in THAI language which I know N800 is not support.

Can I install additional font (THAI) for this unit? If yes
What type of font file can I use?

Appreciated for your help. If can, please give answer in detail because I don't have much experience with this type of portable device

Thank you

I'm not familiar with this so I couldn't help you directly. But I can give you a point that this should be possible, It's Linux based as well. Almost everything is possible then ;-)

It's possible displaying Chinese websites when you have the fonts, so it should also be possible to display Thai sites, once you have the fonts. Of course it all depends on whether the browser knows how to handle Thai encodings. But at least sites encoded in UTF-8 should render correctly.
The N800 needs Truetype fonts, and the Opera browser expects them in a special location (not in .fonts in your home directory). I don't remember the exact location, but you can look it up in this forum somewhere.

Thank you so much