The Local has an account in English of Anatol Stefanowitsch’s sensible defence of JFK: JFK cleared ‘I am a Jam Doughnut’ gaffe
Stefanowitsch said the construction with the article “ein” is used when a speaker wants to say that he doesn’t literally belong to the group, Berliners in this case, but rather wants to express that he has something in common with them.
“That is exactly what Kennedy wanted to do – he did not want to claim to actually be a resident of the city of Berlin but rather to say that he shared something with the Berliners, namely their love of freedom,” Stefanowitsch said.
At the end of his 10-minute address, Kennedy uttered the immortal words: “All free men, wherever they may live, are citizens of Berlin and therefore, as a free man, I take pride in the words ‘Ich bin ein Berliner’.”
So there would have been no blank stares or giggles from the crowd of 450,000 Germans that summer’s day?
“Kennedy not only delivered a grammatically correct sentence but rather the only sentence that made sense there,” Stefanowitsch said.
My only quibble is how Kennedy would have referred to a jam doughnut in American English.