In a European city or town you have knowledge of with a mixed population did many Gentiles have much (or any) Yiddish? Were there periods where Gentiles had more Yiddish than others? Was any understanding of Yiddish limited to commonalities between German languages and Yiddish. Particularly interested in Budapest and Hungarian cities but interested generally.
I cannot speak for the areas in Poland or the Pale, but I can speak to the German experience: For basic communication purposes, spoken Yiddish and spoken German are mutually intelligible - that is to say that if a Yiddish speaking person came across a German speaking person, they would be able to communicate. It would not be perfect by any means, but they could communicate effectively enough for "everyday purposes" like conducting basic business. A perfect example of this is in the Diary of a Napoleonic Footsoldier, in which the protagonist Jakob Walter, a German soldier from Baden fighting in Napoleon's armies, often kidnaps local Jews to serve as his interpreter and guide in Polish towns and villages because they are the only ones he can understand. This is also reflected in the linguistic debate surrounding whether Yiddish is a separate language or merely a dialect of German, so-called Judeo-German.
Sorry I cannot give more info.
Follow-up question: can anyone address the extent of Gentiles' exposure to Yiddish in the United States, back when Yiddish was more widespread as a native language? Would urban moviegoers who had encountered the language in daily life be surprised to see James Cagney speaking it?