If nothing else works, you may want to try this hack:
http://www.pepak.net/files/e-books/p..._hack-0.01.zip
Its flashing process is based on the russification hack which already failed you, but it may happen it will work. Or maybe not. So far I know of one user who bricked his reader using it.
Another thing to try, maybe: You can try to e-mail
support@digiknihy.cz. It's a company which sells readers here in Czech Republic, and a part of their service comprises of adding czech support to their readers. I know they will also update firmware for readers bought abroad. I don't know if they can (and will) unbrick your reader, but I wouldn't be surprised. Of course, it would cost you - first for shipping fees and second for the unbricking itself.