I don't know an answer for that, maybe you could ask for help at the phpBB community. I'm sure they will know what to do.
and how do i change that?
I suggest first trying this before getting too involved in kinds of software:
Type in ideograms (any kind of Chinese)
Does it look weird? If so (based on IE) - do the following...
Click the "view" button in your browser
Then your language (i.e Chinese Simp or Trad).
This problem should now be solved.
i think you may first check if the web server support chinses.
do not do ajax with Chinese encoding (gb2312), it's a nightmare, I'm running into the problem where I can't override the mimetype, hopefully it's something else, but I've been stuck on the same issue forever. XDomainRequest is horrible for doing that, and XmlHttpRequest doesn't have a reliable SetRequestHeaders, and contentType is only available in IE8, cross-browser issues with chinese encoding is terribly stressful, if I could redo my current webpage, I'd go without ajax.