Welcome to MobileRead remazas. (Our official greeter, NatCh, would tip his hat if he was here.
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As for cyrilic on the Reader it is possible and not all that hard. There are two methods that I know of. First is to create special sized PDF files in the Reader's dimensions that have a cyrilic font embedded in them. This will work but pages are fixed in size and the (font) size button does not work for them. Also there are reports that any pages after 1,000 will not be able to be displayed.
The other way (and IMHO the better way) is to create the documents in BookDesigner with embedded cyrilic fonts. These pages reflow when the (font) size button is used. However, the file sizes are larger than when you do not use an embedded font and the time required for page turns increases a bit.
Perhaps they can also be created using libprs500 but I have never looked into that, maybe kovid can answer that one.
Good luck and let us know how it works out for you.