Philalerts, what you are experiencing is probably due to the fact that you have already downloaded the US ROM on your PC, that is why SEUS will install it automatically.
What you need to do is root - root explorer - change build.prop to the pretended version CZ in your case - save!.
Than you shut down the phone, and go to the place were you have your SEUS installed and DELETE the entire db folder "C:\Program Files\Sony Ericsson\Update Service\db
That is where your US ROM is. After that you run SEUS again, and update. You should have a new CZ ROM in your next boot.
Good luck / Hodne stesti (my Cz used to be better