I block and stutter in English(first language) since I was 6 but I don't in my 2nd language. A lot of it I believe has to do with the translation parts of the brain interacting also with the normal speech centres. I just know that when I need to actively translate in my mind, the 'pre thought' is what I'm about to say isn't stuttering. Where as my pre thought in English stutters. Weird to explain, but I know I'm very aware of my stutter when I speak in English, not so much in my 2nd language