I have changed the xml:lang value from "cs_CZ" to "cs-CZ" and everything works fine. Thank you very much for your help.
It's evident that my language specifier didn't follow the RFC 3066. So instead of changing the XmlMind behaviour, which is - in fact - correct, a kind of warning might be useful to indicate wrong language specifiers and potential problems in spell checking. Thanks again Tomas Klinkovsky

