It’s the exact opposite for me. I stutter in every foreign language I speak or try to speak in, including English. I came to the conclusion that 1) I don’t need to think about what and how I’m gonna say what I want in my native language, therefore I’m more fluent and 2) both English and German (that I’m proficient in) has a lot of “dangerous” sounds that make it harder to be fluent. I think it’s more or less the same as in acting. Some stutterers are fluent when they’re acting, I’m much worse because words don’t have a natural flow as a result of citing from a script (and I can’t get around with using synonyms that are easier to pronounce).