This is really something best left to speech therapists/pathologists, I can only speak from personal experience as a stutterer. Feel free to call bullshit, but I think there's a chance the speech impediments we have in our native languages are developed. When we learn another language it is very much a 'forced' thing, for a long time you have to actually think about what to say compared to automatic speech in your native language. As you become more fluent in that language, your brain thinks much in the same way as for your native language. Then you stutter because it's business as usual!