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Vietnamese fonts messed up

Hi guys,
My textbook is for Vietnamese student to learn English and there are a few Vietnamese texts in it. After printing, the texts turned out to be all squares.
Can somebody help me with this?
Many thanks!
If it turned to squares (and not question marks) that suggests a possible encoding error, is the document XML or HTML? Is it encoded in UTF-8?

Which fonts are you using, and which operating system are you running Prince on?