Asuka's father is supposed to be an American (in the anime, at least), which is where the surname "Langley" comes from. If the audience knows that, and she has dual citizenship, then they shouldn't have to explain anything else about why she doesn't have an accent. Occasionally tossing in a German cuss word should be plenty to remind the audience that she's got German in her. ;)
Also, I think it's pretty likely that they won't bother with the whole business of Asuka being 1/4 Japanese, seeing as pretty much the only reason they did it in the anime was so they could give her a Japanese alternate surname (they have a kind of weird attitude about foreign surnames over there). I imagine they'll drop the Soryu part of her name (it wouldn't be the first time *cough*Rebuild*cough*) and possibly replace it with "Zepelin," but doesn't necessarily mean they have to change her first name too. She could just say her dad was a yuppie.
I think it's important that the three pilot's names be left unchanged if at all possible, but the further they move from them, the more flexible I think they could stand to be with the names. For example, I'd prefer Misato's first name to remain unchanged, but if they're not casting an Asian actress for her then it seems reasonable that they'd change her last name. In a treatment I've been writing, for two examples, Kaji's name was changed to "Roger Kimball," and Kensuke was changed to "Kenneth Anderson." I figure that the further you get from the primary cast, the less uncormfortable the fans are with name changes. I speak this from experience, since I myself less comfortable changing the names of main characters than I am with supporting characters.
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