I've been using spark for a while, and can't remember exactly when I might have have upgraded. I used to be able to use Spark to communicate with my MSN (via an openfire plugin) contacts in Mandarin Chinese, but right now in Spark 2.6.3 under linux mint debian edition, any Chinese characters just come up as square boxes.
Other java applications display the Chinese fonts properly, so this isn't an issue with my Java or font installation in general, though maybe Spark is hard-coded to use a font I don't have? I've tried installing more Chinese fonts but that has not fixed the problem.
The square-boxes-instead-of-chinese-characters are visible in both the main contact list, and in chat windows.
The correct unicode data is being transmitted by the server: if I copy/paste chacters from the Spark chat window into another window, they display properly. Also, if I hover over the icon for a Spark chat window on my gnome-panel, I see the properly rendered Chinese characters for the name of my chat contact. The improperly rendered characters are only seen within Spark itself. When typing in a chat window the characters also don't render properly, so the problem has nothing to do with the server side.
I'm pretty confident that this was working at some point, but I can't remember what version I was using before. I'm also experiencing this issue on another computer running Ubuntu 10.10.
If I go to the "Actions" menu and select "Languages," I also see boxes instead of characters for the relevant non-ascii language names.
Under preferences->appearance I see a way to select a font size, but not a way to select a particular font.