This issue has already been raised, discussed, and commented on by Khaled. A duplicate report won't change much; Khaled is aware of the compatibility issues. Also note that the comments in that thread are relevant to what your options are. There will be ways to support your legacy encodings "soon".
It should also be noted that you specifically set your encoding to one that mIRC no longer supports ("CP1251 is your translation scheme"). Perhaps you should consider switching the encoding on the server end to UTF-8. You claim it's not "properly implemented", but I don't know of any specific issues.
As far as "many conditions make migration impossible"-- if your definition of "migration" is continuing to use CP1251, well, we have different definitions of "migration". The only real migration, if what you said about server-side UTF-8 is true, is to get your servers in Russia to properly support Unicode. Until that happens, "migration is impossible"-- but mIRC is not the one holding you back. it's been almost 14 years since Unicode was standardized; it's been by and large the standard on the web for the last ~6. IMHO there's really no reason for mIRC to wait any longer for those servers to get their act together.
