Probably the different sounds. I stutter less in English. Different people have issues with different sounds. English is a "smoother" language than most. Not that many hard open vowels that can lead to blocks. Lots of words in everyday use that start with consonants.Of course for some people it's different and they have issues with sounds like "s" and "t" so English may be worse. I've also read that learned languages use a different part of the brain. Not sure if it's true, but learned languages might be differently than native ones. Apparently billingual people are more prone to stuttering in general.