There is a difference between wanting to make sure somebody understands what you've said and condescending to people who are speaking a second language by constantly interrupting the conversation with "do you understand?"
I went to language school 5 hours a day, 5 days a week for six long months when I first came to this country. I have lived here over 3 years now. I function everyday among people in the German language. I have performed a reprographic internship, gone to art classes, printing classes and birthing classes all in German. I have sat as a mother through two hospitalizations of my child among health professionals communicating with me in German. I have misunderstood, mis-spoken and cried my heart out in attempts to reach understanding with people. I still speak horribly with tons of mistakes. But I understand nearly 90% of everything said around me now and I can make myself understood if given the opportunity.
The greatest gift you can give me is your patience in the GERMAN language instead of telling me that you are GOING to speak english TO me so that YOU are sure that I understand. STOP IT. I'm not your language puppet!