This comment in the code (for Russian) indicates, that fractional numbers are not really supported by Language::convertPlural() method:
Russian, Polish and other slavic languages (and probably many more) use a different form for franctional numbers.
This document:
http://unicode.org/repos/cldr-tmp/trunk/diff/supplemental/language_plural_rules.html#ru
does not seem to address this.
The only case where we *might* needed is to display amount of the funds donated to the Wikimedia Foundation:
(not sure if having fractions of millions there is a good idea, if it is not - please WONTFIX this bug).
Version: 1.18.x
Severity: enhancement
URL: http://svn.wikimedia.org/viewvc/mediawiki/trunk/extensions/CentralNotice/SpecialBannerLoader.php?revision=83744&view=markup#l126