This is very funny, and I could relate to that! I am a Spanish native speaker and naturally stuttered more in Spanish, but not in English nor Japanese! I read somewhere that it all depends on the language pronounciation, if it is not close to the native language, we stutter less. And another thing, in my case, when I lived in Japan, I grew out of my comfort zone and was really confident so I stuttered way less (probably due to confidence, thus breathing better and having good posture), so it might be your case too. At the moment I work as a Japanese interpreter, and tend to stutter more when talking to Japanese bosses, so it must be the confidence part. Anyway, cool to see a fellow gaijin who stutters in Japan. We should grab a beer or two when I'm there!