Ubuntu font cannot display latin extended characters I changed the system font (by Font Changer) to Ubuntu font (0.8). This font has Latin-Extended support and can display Vietnamese just fine (I tested this on http://font.ubuntu.com/). 
However, after I changed to this font, Vietnamese chars are replaced by boxes.

Can anyone explain this for me? Any idea on how to fix this?
I'm not sure about Vietnamese, but for example in Finnish, there are some extended characters that are problematic. This is because some pages are encoded as ISO-8859-15 and some are UTF-8. If autodetection in the browser fails, they will be displayed as boxes or garbage. The same thing happens if the font only supports one encoding and the page uses the other. I'd recommend to check the encoding of the font and the encoding of the pages you tried, and change the font if necessary.