Google Groups module lets you work with Russian messages in a Newsgroup, but not in one of 'legacy' Cyrillic encodings. It sends your message in Unicode(UTF-8) encoding.

It matters for people who would read your message but not via Google Groups page:
those who work with Usenet Newsgroups via newsreader programs, need a modern newsreader software (such as Outlook Express or Mozilla products) which is capable to process UTF-8 texts.

As for reading Cyrillic messages in Newsgroups via Google Groups:
Google Groups keeps all the messages in UTF-8 encoding, that is it always convert non-Unicode messages to Unicode.

This is why sometimes you face UNreadable messages on Google Groups pages.
It means that their authors did not use Google Groups, but rather used traditional newsreader software to post (such as Outlook Express) and did NOT learn how to send non-Western messages in such a way that the encoding of the message is specified in the system header of this message.
When encoding is not specified, then Google Groups has no idea what is the encoding of the text to be converted to UTF-8 and uses "Western European" as a default.
Obviously, conversion  Western-->UTF-8  performed on a Cyrillic text, makes all Cyrillic letters corrupted...

(Cyrillic tune-up instructions for Outlook Express, Mozilla News, Thunderbird are provided in the "Cyrillic(Russian) in Browsers/Mail/News" section of my site)