A stay abroad is not compulsory for studies in the Masters program. However, you are still welcome to study abroad, e.g. through the ERASMUS program. Please note that a one-year study program at a foreign university can more easily be integrated into the Masters program due to its modular structure. Shorter periods, e.g. one semester, are more difficult to integrate, since each module is offered only once per academic year. You might have to wait until the following semester to complete your study in Berlin. Solutions to the latter problem are: a) you find a foreign university with similar unit offers and obtain the respective ECTS credits there, or b) you spend your final semester abroad. Although in this case you would have to postpone your last semester, you would not be obligated to attend additional units.