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.
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).