Emacs now supports ICCCM Extended Segments in X selections.
Some versions of X, notably XFree86, use Extended Segments to encode in X selections characters that belong to character sets which are not part of the list of standard charsets supported by the ICCCM spec. Examples of such non-standard character sets include ISO 8859-14, ISO 8859-15, KOI8-R, and BIG5. To paste selections with such characters into Emacs, use the new coding system compound-text-with-extensions as the value of selection-coding-system.
Yes!!! Finally cut (from any X11 app) & paste (to Emacs) with cyrillic works again! I recall it was broken since I've started using XFree86 4.0.0 and I even somehow managed to get used to it.