Lenny began to rely more and more on what he could do with his voice, hands and facial expressions. […] That discovery was the first step in the direction of abstraction.
The next step was to junk speech in favor of double-talk. Here he was following the lead of Sid Caesar, the greatest double-talk artist in the history of comedy. Sid was a genius with sounds and accents. He couldn’t speak two words of any foreign language, but he would converse for hours in double-talk versions of German, French, Italian, Spanish, Russian, Polish, Japanese—and even more exotic tongues—with such passions and such a flair for the characteristic sounds of these languages that people would swear he was actually speaking the language.
Now, as Lenny realized eventually from his prolonged study of Sid’s stage act, when you make a character speak in double-talk, you actually abstracted the essence of his vocal mannerisms. Once the words were reduced to gibberish, the whole characterization resided in the inflections and tonal peculiarities of the character’s delivery.
Indeed, beyond mere verbal fluency, hand gestures and facial expressions are important aspects of language learning (for the vocabulary of Italian hand gestures, see e.g. here).
Language Log has erudite coverage of doubletalk, with further links. Here’s the famous Sid Caesar routine, with French, German, Italian, and Japanese:
Meanwhile Dario Fo was exploring Grammelot:
By extension, here’s a classic scene from Bananas: