Apple -> System Preferences -> International -> Language -> Edit...
does list Russian (in a Cyrillic font). Lots of other languages are listed too. Some of them aren't installed by default (at least they weren't installed by default on our G5).
I don't know how deep the support is though.
[link|http://web.middlebury.edu/offices/helpdesk/documentation/macs/macos_international.htm|This] 2 page .PDF goes into it a little more.
Cheers,
Scott.