This answer introduced me to Yiddish.
Is Yiddish taught and used in Germany/Austria/Switzerland/Lichtenstein? Is it a dialect of German or a separate but similar language?
I wouldn't call it a dialect like the Bavarian dialect. It is a High German language, derived from Middle High German combined with Hebrew, Aramaic and Slavic.
It was used by the Ashkenazi Jews ("German Jews") living in Germany along the Rhine and spread from there.