Hm, the tool uses utf-8 to read and write files, so Cyrillic should not be an issue 🤔 I tried the translation from the sample game and it worked fine...
Well, you get the .rpy files from RenPy itself by generating translation files... I'm not sure if they let you set the encoding.
I also found this tool that might be able to help you? No idea if it works though.