The possibility of not being understood or not understanding what someone else says often creates tensions on both sides of the conversation.
A good way to diffuse all this discomfort, is to acknowledge the elephant in the room, which will hopefully put both parties more at ease to getting down to business, building rapport and focusing on what is important. You might say something like,
“As you might be able to guess, English is not my first language so I want you to feel comfortable to ask for clarification if you don’t understand anything I say and, similarly, I will ask you any questions if there is something I am unsure about.”
If your client feels confident that you will go away making sure you understand the situation fully and that they are free to ask questions for clarification, you will both have greater confidence in each other and your professionalism will shine.