Honestly ethnolignuistic territories don't form nice neat little chunks with well defined boundaries, especially prior to the modern era. The region known as Prussia is currently split between three countries: Russia, Poland, and Germany. Up until around the first millennium, many ethnolinguistic groups settled here. Gothic, Germanic, Slavic, and even Iranian and Turkic groups have all made their footprint in Prussia at one point or another. But ultimately around the time nationstates and larger settlements started being established the western portion of Prussia was mostly settled by Western Slavs similar to Pomeranians, and the eastern part mostly by Prussians, who were a Baltic group. By the 11th century Germans settlers had already established themselves as a mercantile elite in many places, and it's really hard to say when that process actually started. The Baltic Prussian language fell out of use, but a few loanwords survived in Prussian German, especially "low Prussian". Supposedly there was a minority of native Prussian speakers still existing up until WWII, but the population exchanges and cultural consolidations and enforced linguistic conformity in the region wiped it out in the aftermath of the war. The Pomeranian languages in the Polish part of Prussia were considered to merely be a distinctive accent in Communist Poland, but today are recognized as a separate language that receives limited government support in areas where they form a significant percentage of native speakers.