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Can you hide wired conversations behind a language barrier?
Heavy dialects could be an option to make it at least more difficult for those who listen. My step father is from Southern Germany and speaks a moderate Swabian dialect making it difficult for people in mid Germany to understand everything he says even though the basically speak the same language.
He once had car issues in Hesse and wanted to ask the mechanic if it's okay to leave his car where it's currently parked. What he said was "Ka' i den do stong long?", the generic German equivalent would be "Kann ich den da stehen lassen?".
Assuming both people know the dialect and are able to talk it fast it may be pretty difficult for a third party to understand fast enough what's going on.