If we assume that the Doctor is not speaking in English but rather that everyone hears him in their local language and we just hear him in British English for convenience, what could be the American accent Mr. van Statten hears when the Ninth Doctor speaks? (He made note of Rose being English as soon as he heard her speak, but he didn't seem to notice anything unusual about the Doctor's speech.) I'm not looking necessarily for a "northern" U.S. accent but rather one that would carry similar class connotations and perhaps stereotypes as Christopher Eccleston's accent. (I like to think the Ninth Doctor adopted the slang of the Shobogans out of an instinctual wish to distance himself from the arrogance and massive privilege of the Timelords. From what I can gather, the Tenth Doctor's estuary accent is the stereotype for young upper-class/middle-class people who want to appear more friendly and in touch with the 'common man' by adopting some aspects of the Cockney accent? This would work well with his renewed pride in being a Timelord and a lot of other changes in his general attitude and class-coding.)
Actually, I'm not really looking for what Mr. van Statten would hear, but rather what Jack would hear, assuming he used some kind of futuristic language training device to change his native language to some generic form of 1940s American English. (Having Jack hear the Ninth Doctor speak his actual native language while processing Rose's English at the same time would be too complicated. Plus, he conversed with John Hart in English, too.) So if someone has an idea what the equivalent American accent would have been at the time of World War 2, that would be even better.