1. Install FictionBookDesigner (contrary to BD, FBD provides the full unicode support for the file import).
Installer:
http://www.the-ebook.org/e107/e107_f...okDesigner.zip
Update:
http://www.the-ebook.org/e107/e107_f...2006_11_29.zip
2. Open FBD, assign lang "Polish" and load your file.
3. Make in FBD html0-file (F10)
4. Open BD, assign lang "Czech", load html0-file made with FBD.