I'm not the first to speak about it, but maybe putting things straight will help a little.

Internally, Spiral Knights seems to use Unicode for text representation and supports all symbols included in it (at least if the font can display them). But, the problem is, game on Windows gets codes of the letters pressed on keyboard in ANSI and decodes it using CP-1252. While it works for Western European languages, it fails for others. For example, when typing in Russian, Windows gives codes in CP-1251, but with the game treating it as CP-1252, the text becames completely unreadable. The solution would be to make game accept input in Unicode, or at least to decode it using a encoding specified by system locale.

To note, Spiral Knights handles it correctly on Linux and Mac OS X, the issue is only Windows-related.