If you do not understand your client, do not pretend that you do. Ask the client to repeat what he or she said, then repeat it back to them for confirmation.
If necessary, try to ask questions that require shorter answers or a nod of the head.
Do not speak for the client or attempt to finish his or her sentences.
If you are having difficulty understanding the client, consider writing as an alternative means of communicating. Remember to first ask the client if this is acceptable.