Unicode is the most-used mechanism to go beyond the Latin alphabet and enable computers to use almost any script in use worldwide, be it Cyrillic, Greek, Hebrew, Arabic, Japanese kanji, Chinese hanzi or other still. E.g. on the Internet see the Wikipedia entry for Unicode here.
Symbian fully supports Unicode, and the tool used to build MoStacks on Symbian/UIQ3 (native Symbian C++) also does. If you have the necessary fonts on your phone and the means to input the characters, MoStacks will support non-Latin scripts. However, right-to-left scripts (Hebrew, Arabic) are not tested and maybe won't work, at least not for list boxes.
If you only want to use a stack with non-Latin content, e.g. in the sense of a reference work, without making changes yourself, the question is reduced to using a suitable font. Depending on the locales that your phone supports, you will have to install such a font yourself - see also chapter UIQ3 Fonts. Then configure that font as the stack font.
Windows per se nowadays fully supports Unicode, but nevertheless the MoStacks Windows program supports it only partially. The reason is that the tool used to build it (Visual Basic 6) has only partial support.
You should have the least problems if your PC is running in the so-called DBCS mode which is the case if you run a Japanese or Chinese Windows. In those cases proper Unicode fonts won't be a problem either.
If you want to use non-Latin scripts with MoStacks on Windows with a PC running in a European language like English or German, it gets considerably more difficult. E.g. on Windows XP you have to make sure that Far-east language support is installed, and in your stack use Wiki fields for all fields that have to support non-Latin characters, using a font that contains the necessary characters in the first place, e.g. configured as the stack font.