Russian for Belarusian

I don't think that this is a good idea because it will lead to UI in 3 languages simultaneously.

In the worst case yes, but in such a case isn't it worse to have half of the interface in Cyrillic script (be) and half in Latin script (en)? Ideally the Russian message will not feel too extraneous to the rest of the Belarusian interface

Mix of Russian and Belarusian phrases gives strong feeling of Trasianka (https://en.wikipedia.org/wiki/Trasianka).

Then a mix of Belarusian and English would be a sort of creole. :)