The dialect the good people of Zurich spoke was, and is, in many respects, quite unique (even now). Luther had problems with it and so did The Landgrave of Marburg.
May 7, 1529, [Zwingli wrote the Landgrave in the lead-up to the Marburg Colloquium] – “… that I address you in Latin I do it for this reason only because I fear that our Swiss tongue is strange to you” (viii., 663). So, also, to the same on July 14th he wrote: “I fear that if we meet I shall not be understood in my tongue. So I do not know whether it would not be better if we used Latin” (viii., 324).
At Marburg Luther constantly whined about Zwingli using Greek. He did so not to be a show-off (even though he could have done, being at that stage far better than Luther (though not than Melanchthon) at Greek and the best of the lot in Hebrew) but because the folk there assembled would have been lost had he spoken the language of his home.