I am losing letters due to encoding. I am using czech QWERTY keyboard but this should not be an issue. So far encoding support for files (utf etc.) is OK but dialogs are wrong . Which font is used for dialogs?

I tried Appearance and IDE settings - font settings but these are not related to dialogs.
Thanks for advice.

Check if Windows Control Panel -> Regional and Language Options -> Advanced contains "Czech" in the combobox. If not, please select it and if asked to reuse installed Fonts - do not agree and install them from the Windows CD again.
Hope it helps.

That input you asked about uses "MS Sans Serif" font.

That's amazing, it shows two things. I don't use czech as much as I thought or most programs that I have use unicode
Thanks ddmitrie!

I know MS Sans Serif actually is not a good font for unicode applications, that's why you'd have appropriate MS Sans Serif font installed that has all Czech symbols in your case. The only workaround I found is what I suggested before.