Group discussions are mainly in Deutsch. If you don't how to say something in German, it's no problem if you switch to English for a sec. But you have to try and stick to German. The interview is also in German. So I think they would really like to see that your knowledge of the German language is sufficient. So my advise would be..practise this as much as you can. You can try to read german newspapers, magazines. (for e.g. about recent topics so it's also easier to understand at first). Watch german tv. Just to get used to the language. And of course, if time permits, you can attend a german course. Good way to get your conversation skills at a good level.
Good luck!
