They may fap constantly to hentai, furry porn, or some other crazy Japanese fetish. They also eat lots of California Rolls, Kalamari and Ramen, because those are the only Japanese foods they can think of. They also listen to music ranging from anime soundtracks to bizarre J-Rock.
Sure, it sounds like just a nerd in love with Japanese culture, but when he starts acting like he can speak and write Japanese when he doesn't know jack shit, then he's a Pseudo-Japanese kid.
Guy #2: Pffftt, that kid is Pseudo-Japanese.