I tried to play with Belarus locale to have some fun and noticed ugly mix of Titillium font which is used there for Latin letters, numbers and punctuation and DejavuSans for Cyrillic letters, see screenshot. The very same problem with Cyrillic Serbian.
My proposal is to explicitly set font-family in info.json preferably to Lato(like in Russian) or to DejavuSans(like in Ukrainian). Both fonts support full Cyrillic character set.
Screenshot of Belarus locale. Note numbers and commas