You maybe should have checked that before lecturing a German about German language. What is considered Niederlausitz has changed by time and however since abt. 1796 Beeskow is not anymore in what is considered Niederlausitz or Lausitz.
I just add some arbitrary German maps showing what in German is meant by the term Niederlausitz. I've marked Beeskow in red, mostly with a red ring.
https://i.imgur.com/oBSGIfS.jpg
https://i.imgur.com/C93uZwn.jpg
https://i.imgur.com/ZljCDKc.jpg
https://i.imgur.com/kkUCyS3.jpg
https://i.imgur.com/mWOgIUx.jpg

