International input was fixed before 511 version, however the test platforms are Ubuntu and Fedora...
Are you using 511 (or later snapshot)?
Are you sure that input works in other apps?
Is there something different in Mandriva input method implementation?
(I am using code that is quite similar to one used in other toolkits, there is not much implementation freedom).
Well, there is another problem I am aware of: Many linux fonts (e.g. Bitstream Vera) do not contain czech glyphs. In e.g. KDE apps this is hidden as text rendering subsystem "borrows" missing characters from other fonts. We are planning something similar, but instead of other fonts, we shall "synthetise" characters by combining glyphs (as there are carons etc... defined in both Unicode and Linux fonts).