That's interesting. Do you stammer more even when speaking some English you know well (ie when saying a simple sentence such as "Hello. My name is ...") or is it worse when you are trying out new words and constructions?
One of my students in Spain stammered only when speaking her native language. When speaking English, her stammer vanished. It seemed to work along the same lines as people whose stammer (or even Tourette's syndrome) goes away when they are singing.
Seriously, what happens is, actually, when I decide to speak English whatever what I say - I always stammer. In addition, I also stammer when I speak my native language. I don't know what I should do!!
If you have had this stammer since you first started speaking, when you were very young, then I don't think there is much we can do to assist. In my (non-professional opinion) you need professional help from a speech therapist. A stammer can be rectified at any age but it takes a lot of work and someone who knows what they are doing.