I think you had the same problem as me. for my language (Khmer) is using legacy font and unicode, but i dont know about your language having legacy or not. some of khmer wesite are still using legacy font, so i put legacy font and unicode font manually and it able to read everything, legacy web base built and unicode build. for my suggestion, you can go back to your windows installed fonts folder, then copy all the thai fonts and place into .fonts folder,

i am not sure, but it safe to try.
i meant if your PC can view it, your n900 can view it too. i am pretty sure.

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