Alois,
Sometimes people are not able to find their own posts but start double-posting. In this case admins also sometimes delete this redundant pieces. But in your case here is a LITTLE help to find your “deleted” post from 5 days ago:
http://www.onedesigns.com/support/topic/bad-characters-in-headings-linked-oswald-font
And yes, the Oswald font light is having problems at some sizes with special characters, not only with czech language. Try to switch to another font instead (Appearance/Theme Options/Typography) and play around with the font families.
Cheers,
Alexander @ http://www.mouseclick.com