Having a question about translating phpBB3? Want to discuss and collaborate with people currently translating phpBB3? Here would be the correct place to do so.
Do you see those letters displayed correctly here? If the letter display correctly here, then it is likely a style problem. Try changing the board style to prosilver and see if that resolves the problem. If so, the fonts used by the other style may not support Central European characters (ISO-8859-2). Either find another style or seek support in the specific support forum wherever you got the style from.