Сообщение №1413211092830624

Источников много, но вы тупо посмотрите вики:

In 1260 the Grand Duchy of Lithuania was the land of Lithuania, and ethnic Lithuanians formed majority (67.5%) of its 0.4 million population. With the acquisition of new Ruthenian territories, in 1340 this portion decreased to 30%. By the time of the largest expansion towards Rus' lands, which came at the end of the 13th and during the 14th century, the territory of the Grand Duchy of Lithuania was 800 to 930 thousand km2, just 10% to 14% of which was ethnically Lithuanian.

Ruthenian - это название русских которое было определено уже много позже разделения. Расхождение же по языку началось только в 14 веке и закончилось к середине 17. 3 века спустя.
This linguistic divergence is confirmed by the need for translators during the mid-17th-century negotiations for the Treaty of Pereyaslav, between Bohdan Khmelnytsky, ruler of the Zaporozhian Host, and the Russian state.