Latvian language
Latvian ("Latviešu valoda" ) is a Baltic language spoken in the Baltic region. It is the language of Latvians and the official language of Latvia as well as one of the official languages of the European Union. It is sometimes known in English as Lettish, and cognates of the word remain the most commonly used name for the Latvian language in Germanic languages other than English. There are about 1.3 million native Latvian speakers in Latvia and 100,000 abroad. Altogether, 2 million, or 80% of the population of Latvia, speak Latvian. Of those, 1.16 million or 56% use it as their primary language at home. The use of the Latvian language in various areas of social life in Latvia is increasing.
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.