i found
only one way to coming back to a working backend, after the user select this CS-Language and saved the settings
repaired directly in the database or a fresh install (if everything is empty (means: after a new install)

1. in both cases use the repaired file in the attachement, unpack the zip and overwrite the file in the folder "languages"
for a fresh install...2. delete everything in the file config.php and leave their only this row
<?php
3. start the installer again (simple call your domain in your browser)
repair the databaseuse a database tool like phpmyadmin or mysqldumper
go to table
addons and sort it by type or name
search for the item with name==Česky
edit this line and be sure, that the entry for "directory" == CS
save your edit's
go to table
userssearch there for the row language
edit this line, where you have there a CZ and rename this CZ to CS
save your edit's
on my local testarea i have to clear the session datas now and after that, its possible to use the czechoslovakian language in the backend
P.S. not everything in the language file is translated and visible in english. Feel free to help us and post some missing translations from time to time