I know there were no Belarusians and Ukrainians back then. I was using the terms for simplicity. The official language of the GDL was руська мова based on dialects spoken on territories of present day Belarus. Not all Belarusians identified themselves Rusyns. Some adopted identity litvin. Rusyns lived in Polessie and estern Belarus – on the other side of Biarezna river. Litvins in central, ,western and north-western Belarus.