I stutter in French but not in English, or Japanese. Why?

I stutter in French but not in English, or Japanese. Why? Hello. My native language is French and I've always had a stuttering problem speaking it. Nothing too intense, I can manage most sentences by now (spent a couple of months doing therapy while in middle school), but I still speak quite fast (although I don't always realize it), can't manage my breathing properly, and legitimately cannot say some sentences unless I speak *really* slowly. In short, it doesn't make my life a nightmare by any means but it is there and it's preventing me to be myself often enough for me to never really forget I have this condition. ​ Now that's when it gets weird. 2 years ago I moved to Japan, and since then I'm living in an international dormitory. So now I spend most of my time speaking English. Sometimes Japanese but I'm not fluent with it yet so I'd rather not consider it, as me not stuttering in Japanese has everything to do with me speaking slowly since I have trouble coming up with vocabulary. But I consider myself almost native with English by now, I don't even have a French accent (or barely), and yet I don't stutter *at all.* Or let's say, just as much as your regular guy. ​ I am really happy about this, and at the same time quite frustrated that it's easier for me to be myself in my second language than my first. How is this possible? Stuttering has to do with my breathing system, not the language I speak, so I have trouble explaining this. I get that French is a thousand times more difficult to pronounce than English, but is that really all there is to it? ​ Thank you!