I usually go with a variety of surnames. I have a German surname myself, so I'm open to using surnames that aren't English or Japanese. In one of my fanfics, Morty's last name is Lenoir, which is French. Another fanfic of mine lists Hilbert's surname as Klump, which is German. And I make liberal use of Italian surnames (especially Sicilian) because I'm three-quarters Italian (for example, I gave Nate the surname Todaro - a Sicilian surname). The weirdest last name I used was on Grimsley. In my fanfics, his last name is Boros - a Hungarian last name. The same story lists his one of his two middle names as Attila, which is in reference to Attila the Hun (and it's also a common name in Hungary). I chose Hungarian because I think the language sounds cool.