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By the Middle Ages, large numbers of Jews lived in the Holy Roman Empire and had assimilated into German culture, including many Jews who had previously assimilated into French culture and had spoken a mixed Judeo-French language. Upon assimilating into German culture, the Jewish German peoples incorporated major parts of the German language and elements of other European languages into a mixed language known as Yiddish. However tolerance and assimilation of Jews in German society suddenly ended during the Crusades with many Jews being forcefully expelled from Germany and Western Yiddish disappeared as a language in Germany over the centuries, with German Jewish people fully adopting the German language.

By what time period had many of the Jewish  population joined the Holy Roman Empire?
Middle Ages