So all this time I had thought Yiddish was an Eastern European language, based on German. Turns out Yiddish was developed in Central Europe, especially Germany. This makes sense of course. Children learn the language of their host country. I know children of Olim that speak poor English. When Jews went to Russia, they wouldn't develop Yiddish, a Germanic language just because they might have spoken German. They would have learned Russian. So what happened is that when Jews came to Germany or let's call it the Rhine Valley, they learned German and turned it into Yiddish. When they went to Russia, they went with an intact language that actually changed a bit into Eastern Yiddish. In Germany they spoke Western Yiddish.
After the emancipation most of the Jews of Germany switched to German much as children of Russia immigrants in America switched from Yiddish to English.
Everywhere Jews went they put a Jewish spin on the host language. So we have records of Judeo-Latin and Judeo-Romance languages. We see one of the words in the Yiddish 'bentch.' This is not from German where the verb 'to bentch' is zu segnen. The word comes from the Latin ut benedicam.
One sees the various influences on Yiddish in the following simple sentence:
קומען קינדער בענטשן און עסן די כריין
Come children, make a blessing and eat the horseradish.
Come is Hebrew, kinder is German, bless is Latin, and is German, eat is German, the is German, and horseradish (chrayn) is Ukrainian/Slavic.