Looks like my little problem is now fixed
Well, I finally checked what fonts is fbreader using to render the Windows-1250 encoded document...well I think you can already guess what was wrong
Fbreader was using "Droid Sans Fallback" and it seems that this font does not have the needed glyphs for the czech language
After changing it to "Droid Sans", it renders correctly and I see no more squares
A quick "How to display czech text (Windows-1250) in FBReader on SmartQ7@V5 firmware":
1) start FBReader
2) go to options
3) select the "Styles" tab
4) in the family box, select "Droid Sans"
5) press Apply
(you might have to choose it for the other types than the default "Base" too)
Thats it
PS.: Sorry for hijacking your thread a bit, rogue_ronin