Nope, I don't believe this. I learned Spanish in middle school and high school and stuttered about equally in both Spanish and my native English. My stuttering didn't change as a result of it. Interestingly though, I am ethnically Chinese and my stutter gets worse when I'm speaking in Chinese (my second language). When I was a kid, my speech pathologist actually told my parents that stutterers who were raised to be bilingual have worse stutters, so they put an end to my Chinese lessons.