The first general meeting of the conference occurred Saturday, October 2. At the very outset Luther gained an important advantage over Zwingli when, in deference to Luther’s wish, it was decided that the colloquy should be conducted in the German language. Compelled to rely upon his Swiss-German, Zwingli found himself seriously handicapped in the discussions, for the dialect which he spoke differed so from the German of his opponents that he found it difficult to understand and to make himself understood. He had all along hoped that the colloquy would be conducted in Latin.*
Swiss German in the 16th century was quite a bit different than Luther’s German. Here’s a snippet of Zwingli’s dialect:
Instruction uff die frommen, vesten, fürsichtigen unnd wysen unnser liebe getrüwen Alt Burgermeyster unnd mitträth herren Diethelm Röysten unnd Vly Fungken, was sy yetzt uff dem burgertag zuo Arow deß Straßburgischen burgrechten unnd anderer verständtnisen halb handlen unnd fürtragen söllenn, inen von Burgermeyster, den oberisten meysteren unnd heymlich verordneten räthen der statt Zürich zehandlen bevolchenn.
And here’s some Luther:
Gnad und frid von Gott unserm vater und herrn Ihesu Christo. Fuersichtigen weysen lieben herrn. Wie wol ich nu wol drey jar verbannet und ynn die acht gethan hette sollen schweygen, wo ich menschen gepott mehr denn Gott geschewet hett, wie denn auch viel ynn deutschen landen beyde gros und kleyn mein reden und schreiben aus der selben sach noch ymer verfolgen und viel blutts drueber vergiessen.
The use of such divergent dialects of German at Marburg was frustrating in the extreme to Zwingli, who frequently switched to Greek; and that annoyed Luther, who thought he was doing it to show off but he was simply striving to express himself in such a way that Luther and the others (all of whom knew Greek and Latin- it was their odd German which obfuscated things) could understand him.
Communication requires comprehension. Things may have gone quite differently if the discussions had been held in Latin. But, stubborn as he was, Luther wanted it in German…
S. Simpson, Life of Ulrich Zwingli: The Swiss Patriot and Reformer (pp. 190–191).