I showed my Church Slavonic prayerbook, written in Latin characters to a Polish guy I know. He could read the Church Slavonic without much difficulty.
The Polish script is not that easy. We had to learn it for a performance of Pendereski's version. Damn fine piece, by the way...
The challenge is that while Polish LOOKS like English, it certainly doesn't SOUND like English. We wound up with a transliteration. Just like French, there are a lot of letters which nobody seems to use.