Well for me it is opposite. mine get worse when I switch to English, cuz it is my second language not the native. I am more comfortable in speaking in my native language and I hardly stutter with it. I think it has to do with my anxiety since English is used as official language at work where I am most of the time under stress and in pressure . So to sum up: office stressful environment + anxiety of making mistakes while speaking English+ not good with vocabulary thus can't switch words = > stutter