Why do I stutter in one language but talk completely normally in another?

Why do I stutter in one language but talk completely normally in another? When I talk in English, I experience no stuttering at all. It has never been an issue for me. However, when I speak in my native language (Arabic), I stutter a lot, to the point where I refuse to talk because I know I'm going to butcher whatever I'm going to attempt to say. Another thing I've noticed about my stuttering is that the difficulty lies in starting a sentence. I find it very difficult to start talking, but once I'm in the middle of the sentence and achieved a rhythm I won't stutter. I've also noticed that the difficulty in starting a sentence depends on the sound of the beginning of the sentence. For example, I rarely have issues in starting a sentence that begins with "a." But I experience a lot more difficulty in starting a sentence that begins with the "sh" sound. So my question is why do I stutter more in Arabic than English, and why does the severity of my stutter depend on the sound of the start of the sentence? Thanks.