I entirely agree with Tdol. After living in Spain for about four months, I was invited to a party where I was one of only two English-speakers. Everyone else was Spanish and, quite reasonably, either only spoke Spanish or chose to only speak Spanish that evening. My Spanish, at that point, was pretty terrible. My vocabulary was great but my grammar was a disaster. I could only use the present tense and I constantly messed up adjective endings. However, I realised that I had the choice between saying nothing all evening and just listening to the others, or attempting to join in. With the aid of a couple of large glasses of rioja (!), I joined in. To my surprise, the Spaniards understood almost everything I said! The grammar might have been wrong most of the time but the meaning behind what I was saying clearly came across perfectly well.
So - just do it! Open your mouth, talk and surprise yourself!