Why does stutter affect one language but does not affect the other?

Why does stutter affect one language but does not affect the other? Hello everyone. I too have stutter and I noticed that whenever I speak my first language I stutter while when I speak English I don't. With English, for some reason, I can be really concise and talk really confidently about whatever I want. Has this happened to you? Does anyone have an explanation to this? Thank you.