Take it easy. Sure there is a learning curve when you plunge into a new project, your client will reasonablely give you some time to understand their problem. ;) They hired you for your knowledge, not your language. If you fell uncomfortable with your communication, then you could try to speak less and always try to hit the point. If you could solve the problem and maintain good relationship with the client, who cares what the fxxk u are talking about, lol ;)
stay loose. pressure is a good tutor who will guide a tough guy fast move forwards. trust your learning skill and keep a good relationship with your boss & client. then what is the worst result?