He had an interpreter, Heinz Weber, who worked for the German Auswärtiges Amt/Foreign Ministry. Weber was the replacement for Robert Lochner, a journalist (head of RIAS), bilingual but not a professional interpreter, who had interpreted for Kennedy in Cologne, Bonn and Frankfurt. He botched a few sentences and wasn't able to convey Kennedys dry wit. Lochner was also the one who taught Kennedy how to pronounce "ich bin ein Berliner". This article by the German weekly der Spiegel, published a week after Kennedy's visit to Berlin, talks about it. https://www.spiegel.de/spiegel/print/d-45144054.html It's obviously in German but in this video of the speech you can hear Kennedy thank Weber for translating (at about 3:16) https://www.youtube.com/watch?v=JmjeiIMIOs4