I have read an article about this phenomenon (Though I cannot for my life remember where). Essentially, since you are using a different part of your brain, and different neurological pathways, when you speak a second or third language, you are tricking your own brain. However, in some cases (but certainly not all of them), after a few months/years of speaking another language, the brain catches on, and the stutter might start showing up in these languages as well. In short, the brain is amazingly complex, and learning another language is awesome. :)