I have the same problem with Czech ebooks (same coding like Polish).
Anyway, IMHO it is not problem of Calibre, but problem of iRiver Story only. I tried the set of free books from Municipal library of Prague (
http://www.mlp.cz/karelcapek/). Books should be created by Lexcycle Stanza v. 1.0.0-beta16.
I will ask the iRiver for solution of this problem...