Sorry about the occupation - I had no part of it, as I was born only in 1969. And I was never a Communist believer, although I was a "Young Pioneer" and a Comsomol member for a couple of years in high school, when everyone was required to join.
As far as
adding a new language, it is not very simple. The languages and keyboards are "hardcoded" in the JS bytecode, and they are quite a pain to extend. I replaced Dutch with Russian - that was the easiest thing to do. You can replace German or French with Czech, if you wish to keep Russian. I think your Czech compatriots will appreciate if you replace Russian with Czech and keep the other three languages. Getting rid of the Russian menu will not preclude you from reading Russian books, of course.
You would have to unpack the FW image and do all the changes yourself. The Chinese and the Thai are already running with it, so you should be able to, if you want to.
As far as changing the default codepage for the non-unicode files, you can do it yourself (I
provided the source code for the little DLL that modifies the default codepage to Win1251).
The fonts in my version already include East European character sets, so you shouldn't need to replace those. But you have to be very careful with loading new images into the PRS-600, especially if you don't have experience with making FW images for Sony Readers. The 10-reset method of forcing the Recovery mode doesn't work on PRS-600 (for some reason Sony didn't include it in their bootloader this time). The firmware supports some autorun, so you should test your images before flashing.
Here is how to do the autorun testing:
1) You need to install one of my firmwares
2) You need to copy the autorun files (
test directory) into the Reader's main storage and reboot the Reader
3) The autorun should then be redirected to the
/Data/test
4) If you insert one of the autorun cards (an SD card that has a
Sony Reader/software/autorun.xml file on it, doesn't matter what's in this file) while holding the Vol+ and Home buttons, the Reader should execute autorun menu from the /Data/test directory.
5) The only option there that works is (+) - Load test firmware.
Best of luck to you!