Latvian language
The history of the Latvian language (cf. below) has placed it in a peculiar position for a language of its size whereby it is spoken by a large number of non-native speakers as compared to native speakers. The immigrant and minority population in Latvia is 700,000 people: Russians, Belarusians, Ukrainians, Poles, and others. The majority of immigrants came to Latvia between 1940–1991; supplementing pre-existing ethnic minority communities (Latvian Germans, Latvian Jews). In a recent survey, 60% of Latvia's ethnic minorities described their knowledge of Latvian as fluent. Fluency in Latvian is prevalent among the younger generations of the minorities.
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.