From the Wikipedia page on Sid Caesar
Max and Ida Caesar ran a restaurant, a 24-hour luncheonette. By waiting on tables, their son learned to mimic the patois, rhythm and accents of the diverse clientele, a technique he termed double-talk, which he used throughout his career. He first tried double-talk with a group of Italians, his head barely reaching above the table. They enjoyed it so much that they sent him over to a group of Poles to repeat his native-sounding patter in Polish, and so on with Russians, Hungarians, Frenchmen, Spaniards, Lithuanians, and Bulgarians.
Of his double-talk routines, Carl Reiner said, "His ability to doubletalk every language known to man was impeccable," and during one performance Caesar imitated four different languages but with almost no real words. Despite his apparent fluency in many languages, Caesar could actually speak only English and Yiddish. In 2008, Caesar told a USA Today reporter, "Every language has its own music ... If you listen to a language for 15 minutes, you know the rhythm and song." Having developed this mimicry skill, he could create entire monologues using gibberish in numerous languages, as he did in a skit in which he played a German general.