the left side of the brain mostly the side where language is processed. It's interesting because my wife was born Israel and she moved to the US when she was 5 - and she does stutter in her native Hebrew and English. However, I am from Poland - I speak both English and Polish (I don't stutter) and she learned some phrases in Polish from me (a language she wasn't familiar with) and she doesn't stutter when she speaks a little Polish.