Latvian language
Upper Latvian dialect is spoken in Eastern Latvia. It is set apart from rest of the Latvian by number of phonetic differences. The dialect has two main varieties – Selonian and Non-Selonian. There is a standard language, the Latgalian language, which is based on deep Non-Selonian varieties spoken in south of Latgale. The term "Latgalian" is sometimes also applied to all Non-Selonian varieties or even the whole dialect. However, it is unclear if it is accurate to use the term for any varieties besides the standard language. While the term may refer to varieties spoken in Latgale or by Latgalians, not all speakers identify as speaking Latgalian, for example, speakers of deep Non-Selonian varieties in Vidzeme explicitly deny speaking Latgalian.
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.