Hi dregore,
This issue with Cyrillic characters was recently noted as a side issue in
another thread regarding a different issue about Kobo pop-up footnotes. If you look at posts #25 and #26 you will see that Semwize had a solution to your problem by removing a couple of the Kobo built-in system fonts. However, I don't know how he achieved this. If no-one posts details here perhaps you could contact Semwize directly via PM to ask him how he did it.