Earlier quoted context omitted.
No. How did you arrive at that conclusion? The equivalent for Italian would be colonization of some area (say, somewhere around the Mediterranean or Black Sea) by Venetians and other Adriatic or Northern Italic peoples, founding new urban centers, developing a mish-mash of their original Romance dialects, and exerting cultural influence via their more advanced culture/technology ; and then some local non-Italic peopl…
Germanic tribes did colonize northern Italy and eventually used the local latin language with heavy borrowings from their original language. Not sure why you used the word "colonizer" for Jews living in Germany. Perhaps I misunderstood, but given the book you referenced I made an educated guess
I didn't. Rather, "Germans" (Suebians, Saxons, etc) settled in ("colonized") Eastern Europe, beyond the Empire, typically at the invitation of the local rulers. https://de.wikipedia.org/wiki/Hochmittelalterliche_Ostsiedlu... That's where the contact with Eastern Jews happened, which, according to Koestler, gave rise to Yiddish.
Yes, the Langobards and the Goths settling in Italy and Spain lost their language, but the medieval Germans settling in Eastern Europe a couple hundred years later did not.