Around AD 1100, persecuted Ashkenazi Jews established a secret code by writing to one another in German but using Hebrew letters. Hebrew speakers did not know the German words, and German speakers did not know the Hebrew letters. Even today Yiddish is closer to Middle High German than to modern German. In the United States, Yiddish helped tie together the Ashkenazi immigrant communities of a century ago but, as they assimilated, they took up English. Today in the United States it is spoken primarily by a small number of theologically conservative Jews, most of them in the New York City area.
A personal recollection here. My parents spoke Yiddish, and so during my childhood I picked up some of the language. Once, during my teen years, I was on a New York City subway train when a woman approached and asked me directions in what I recognized as Yiddish. I gave her the directions she needed. As we were parting, I said to her, “I’m glad to meet you. Yiddish is not much spoken these days.” She replied, “What Yiddish? I speak only German.” Yiddish tends to be an older German but the languages are that similar.