I would say that as it's written by an American writer, the way the non-English characters speak could be seen to be the default accent from the American perspective... BUT Paul Cornell did a similar thing with various English accents in his Knight and Squire comics (for example, the Somerset issue) and Grant Morrison's Batman and Robin had villains that spoke with phonetic Geordie and Cockney accents.

Maybe it's because my grandad is from up that way, but the idea of a Geordie supervillain really cracks me up. ^^