Q:

HTML: How do i debug why a language does not display correctly

i was recently asked why a tumblr theme of mine does not display Vietnamese correctly on this site. how do i debug whats the problem. 

i wonder if its because of the use of
a custom font or cufon? 
maybe its a character set issue? but
UTF-8 shld support most languages?

A:

Debugging is difficult, especially if you don't read the language in question. There are some things you should check though:
1.) Fonts. This is the main cause of trouble. If you want to display a character you must have that character in the selected font. If you use standard fonts that may work on internationalised Windows but there are also "unicode" fonts (ie, Arial Unicode MS) you may want to specify explicitly.
2.) Encoding. Make sure the page is served in an appropriate character set. Check the HTTP and HTML headers "charset". UTF-8 is appropriate for most languages.
3.) Browser and OS Support. It's pretty much a given these days that browsers support non-latin character sets, however it's possible the client has a very old or unusual browser. Can't hurt to find out which browser/os combination they are using and what their "Regional Settings" are.

