Slovincian and Kashubian
During the High Middle Ages, German immigration and assimilation of the Slavic Pomeranians (Ostsiedlung) introduced Low German Pomeranian dialects which were to become dominant in Pomerania except for some areas in the East, where the populace remained largely Slavic and continued the use of the Slavic Pomeranian language. This was especially the case in Pomerelia, where the Slavic population became known as Kashubians and their language accordingly as Kashubian. An insular Slavic Pomeranian dialect spoken northwest of Kashubia until the 20th century became known as Slovincian. It is disputed whether Slovincian may be regarded as a dialect of Kashubian or a separate language. Likewise, it is disputed whether Kashubian may be regarded as a dialect of Polish or a separate language.
Read more about this topic: Pomeranian Language