This kind of test really depends on the university. So the lz mm needs to check with the office in charge of oral test about the format and their expectation. The one in our Univ. is a very formal class with assigned topic. The judges are professors and students with different backgrounds and will ask all kind of weird questions. Language is important, but they care more about the interaction between the TA and students and how TA can respond to any unexpected question. Basically, you should lead the class rather than be led by questions from students. If you don't know the answer or cannot understand their questions, just be frank rather than guessing.