A US (FAA) Captain of an N-reg Boeing can fly wherever he wants, just as I (FAA PPL) can fly wherever I want with an N-reg. The real question is: can I (FAA PPL) fly in Spain or France with a G-reg a/c? Let's say the US (FAA) Capt is flying a D-aircraft over France. Would that be tolerated?
As for France: I know a Dutch Pilot who flies a F-reg a/c with a FAA PPL. This is according to what is mentioned earlier: you can fly with a FAA PPL on a F-reg plane if you're not a resident and for VFR only. The thing is that I recently wrote the French about this but they said I had to convert FAA -> JAA. I wrote back and told them what I am writing here, but all I got is silence since then.