It is possible to add a new font at least. Go to the world panel, tap the name of the world to open the settings, tap on "font settings," then "import new font." Mosi uses the bitsyfont format, so you can search itch.io for those or make your own with Fontsy: https://seansleblanc.itch.io/fontsy

A quick look doesn't turn up any Cyrillic fonts, but I see Arabic, Thai, and Extended Latin options.