Are the Belarusian and Russian languages very similar?
Although both languages belong to the same group of Slavic languages, the differences between the two are significant. Russians cannot understand the Belarusian language well, especially if authentic Belarusian words are used. Belarusians can understand Russian, however, because almost all of them are bilingual.
The old Belarusian language was an official language of the Belarusian–Lithuanian State Grand Duchy of Lithuania (1230–1596). In 1517, the great Belarusian scholar from Polatsk, Doctor Francysc Skaryna, published the Bible in the Belarusian language. The Belarusians became the third nation after the Germans and the Czechs to have a printed Bible in their native language.
https://belarusdigest.com/story/myths/are-the-belarusian-and-russian-languages-very-similar/