Version: (using KDE KDE 3.5.0) When you start kde for the first time, kpersonalizer asks you to choose which language to use. If you select say Slovak, the dialogs are in slovak but some characters are missing from text because of bad font selected. Bitstream Vera is not the best solution for every language. Either change the fonts according to selected langugage or make the default font some other. E.g. Dejavu is extended version of Bitsream Vera so it looks the same but has more characters included. http://dejavu.sourceforge.net/wiki/index.php/Main_Page
Qt is supposed to look into other fonts when the selected font doesn't have the requested character; so supposedly "characters missing because bad font selected" shouldn't happen. If it does, it's a Qt bug...
it's a long standing one, Qt3 only cares for "scripts" and latin is one for it. So if a font does not have all of latin characters, Qt3 will miss them. Qt4 works different here. But what you really should do, is fixing your fontconfig setup (/etc/fonts usually) - KDE's default is "sans serif" not Bitstream something. And fontconfig _does_ care for the current locale