Latgalian language
Latgalian is spoken in Latgale, the eastern part of Latvia. It is debated whether it is a separate language or a distinct dialect of Latvian. Nevertheless, its standardized form is recognized and protected as a "historical variety of the Latvian language" "(vēsturisks latviešu valodas paveids)" by Latvian law. The 2011 Latvian census established that 8.8% of Latvia's inhabitants, or 164,500 people, speak Latgalian daily. 97,600 of them live in Latgale, 29,400 in Riga and 14,400 in the Riga region.
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.