History /
When will you Poles give back German land and the cities which you robbed? [557]
It's hard to image a number of languages jumping out all of a sudden from nowhere at once.
They surely didn't!
We all just should not forget that a language that is not standardized - like all European languages exept Latin and Greek were 2,000 years ago - just had tremendously fast mutation rates. Languages could change so fast when there were given from generation to generation that you could compare it to the game "Chinese whispers" (German: "Stille Post", Polish: "Głuchy telefon"). Every person in the game that gets a word whispered is like one generation. So after only 100 years or five generations a language could be transformed to an almost incomprehensable new form.
Since the dawn of mass media and standardization most of the European languages got frozen, mostly in the form they had in the 1750s. An average German surely can master a written German text from the 1760s while the same text in the Middle High German, spoken 1360 might be very difficult to understand.
So Germanic and Slavic languages may sound very different now but this does not mean they were not much more similar 3.000 years ago. Everyone can see that Slavic and Germanic languages still have striking similarities when it comes to basic words:
Milch/milk/mleko
Wasser/water/woda
Mühle/mill/młyn
Schwester/sister/siostra
zwei/two/dwa
You could go one and go one with that list.
So why making a dichotomy between Germanic and Slavic languages instead of accepting these language groups still are relatively close related?
Did you know that one of the Polabian Slavic languages survived as long as until the 16th (or even 17th century) and it was west of the river ELBE, while other Polabian languages between Elbe and Oder vanished much earlier?
The last speaker with Polabian as mother language died in 1756, the last person with some limited knowledge of Polabian died in 1825, according to Wikipedia. As you noted those speakers lived in the very west of the former Polabian language area, in an area in Lower Saxony called "Wendland". Note: "Wende" is the old German word for Slav.
Of course the last forms of polabian were heavily Germanized. Just check the Polabian Lord's prayer:
en.wikipedia.org/wiki/Polabian_language