Latgalian language
Originally Latgalians were a tribe living in modern Vidzeme and Latgale. It is thought that they spoke the Latvian language, which later spread through the rest of modern Latvia, absorbing features of the Old Curonian, Semigallian, Selonian and Livonian languages. The Latgale area became politically separated during the Polish–Swedish wars, remaining part of the Polish–Lithuanian Commonwealth as the Inflanty Voivodeship, while the rest of Latvians lived in lands dominated by Baltic German nobility. Both centuries of separate development and the influence of different prestige languages likely contributed to the development of modern Latgalian as distinct from the language spoken in Vidzeme and other parts of Latvia.
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.