You can indicate in which city you would like to do an internship. In the registration form (that will be send through e-mail) you can indicate which three cities / countries have your preference. Try to arrange these destinations in order from your most preferred option to the least preferred one.
If you have just one location, then you obviously only fill in this location. Have you not chosen a specific destination yet? Then you can leave the answer to this question vacant and we will try to help you make a decision that suits you best. We will keep multiple things in mind to make the best decision for you, including where we have the most/best contacts, open vacancies and the sector you would like to complete an internship in.