The Kobo implementation seems to be very bare bones, but any device with mobile Adobe Digital Editions should be able to read Russian ebooks if a Russian capable font is embedded into the ebook. This is the case for both PDF and ePub. See for example
How to embed fonts in ePub after Calibre.
One some devices, it is possible to add fonts on the device and reference them in an ePub (so there is one copy per device, rather than one copy of the font in each ePub). We don't know how to do this on the Kobo yet though.