Language fallback
My rant about Ukrainian was sarcastic, sorry. You should request a comment from the communities (be and be-tarask) regarding the change. Ukrainian is not an option here, only Russian.
My educated guess is: 1) the overwhelming majority of readers, and quite a portion of contributors would find that comfortable, and be silent, and never say 'thank you'; 2) certain vocal minority, while also arguably being more comfortable, would pointedly demonstrate their discontent, possibly even fill your inbox with hate-mail. You choose :).
Looks like you're right. ;-) I also see that 91 % of traffic from Belarus goes to the Russian Wikipedia; only 1 % ends up visiting the Belarusian Wikipedia. https://stats.wikimedia.org/wikimedia/squids/SquidReportPageViewsPerCountryBreakdownHuge.htm On the other hand, only 20 % of the traffic to the Belarusian Wikipedia comes from Belarus! https://stats.wikimedia.org/wikimedia/squids/SquidReportPageViewsPerLanguageBreakdown.htm#Belarusian
So it seems extremely likely that users of the Belarusian interface are equally comfortable with Russian.