It's not the problem with displaying all of the fonts, it is only about some "non-standard" ones (as for me) like hindi etc. I've all needed fonts installed.
Here's screen of wiki page with all languages:
As you can see, every font is displayed properly, but when I copypaste text to xchat, i get either squares or I don't see anything:
http://puu.sh/yRi <- xchat 2.8.9 on win7 x64
I even installed mirc to check if it is maybe some problem with installed fonts, but in mirc I see everything just fine:
http://puu.sh/yRk <- mirc 7.15 on win7 x64
I thought that's problem with xchat, so i tested xchat on linux, but it's rendering all fonts without any problem:
http://puu.sh/yRl <- xchat 2.8.8-3 (from debian repo) on debian 5 (in vmware)
I've few unicode fonts like arial unicode ms, code2000/2001/2002, bitstream cyberbit + cjk, everson mono and unifont which should be enough to display almost every language on earth, but still xchat on win32 is not using them (eg. xchat is using old ms gothic font [which was default japanese font for windows 2000/xp] instead of meyrio font [default for vista/7]).
I really think win32 xchat is handicapped compared to linux version when it comes to displaying fonts, and maybe you know how to change it, or maybe there is some workaround to display all of the fonts
and btw, in every client I used same font - comfortaa, and xchats are set to IRC charset