Latvian language
The Middle dialect is spoken in central and Southwestern Latvia. Kursenieki language, which used to be spoken along Curonian Spit, is closely related to the varieties of middle dialect spoken in Courland. The dialect is divided into the Vidzeme variety, the Curonian variety and the Semigallian variety. The Vidzeme variety and the Semigallian variety are closer to each other than to the Curonian variety, which is more archaic than the other two. There are three intonations in the Middle dialect. In the Semigallian variety, "ŗ" is still used. Standard Latvian is based on the middle dialect.
Demographic history of the Vilnius region
Since the first contact in the 9th century the Slavic (Ruthenian, later Ukrainian) speaking areas have always bordered the vicinity of eastern contemporary Lithuania. Towards the 16th century the Lithuanian language began to become replaced by the Slavic languages. A large number of the Lithuanian speaking population began to become increasingly Slavicized. But still survived in certain parts of the region even up until the end of the 19th century and onwards. The common Slavic dialect spoken in Vilnius region was known as "mowa prosta" and was similar to both Belarusian and Polish. 
In 2015 Polish linguist Miroslaw Jankowiak attested that many Poles of Vilnius region still speak the language which they call "mowa prosta" ('simple speech') by local people.