As is seen in many U.S. films and TV shows, the use of the foreign language and foreign accents by characters is inconsistent, particularly in the courtroom scene. All the court officials are German but spoke English with a German accent. There is no indication in the story line that they would speak English as an accommodation to Perry - in fact, they weren't very accommodating to Perry at all. On the witness stand, Frau Zimmer responded to one question in German but to others in English with a German accent. And while he was on the witness stand Professor Ritter spoke in English with an accent - even while he appeared to be losing his mind - but sang in German.
At the start the woman and child get into a VW bug with left hand drive as is used in America. In East Germany it would have right hand drive.
At around the midpoint, Samuel and Perry pass a phone booth which has the word "telephon" (telephone less the last 'e') in neon above it. In Europe it would be spelled "telefon" & not "telephon"...
There is no glass in the phone booth doors.
When the supposedly native German double agent returns to the hotel room to call his superiors, he uses a decidedly American expression: "Dog gone it!"
The final scene is supposed to take place at a Hotel in Germany, but a Porsche car with California license plates is plainly visible.