Most programming interviews involve simple questions which infer whether or not you understand basic programming concepts like pointers, recursion, memory leaks, streams. Since you're applying for a trainee position, you may receive none of these questions, and even if they do, I'm sure it is just to get a basic understanding of your level, so don't stress it too much.
However, if you have a couple hours time on your hands, I would read up on basic Java and C# language questions so that if it did come up, you would have an answer to simple questions. If they ask a question that you don't know the answer to, don't be afraid to say you don't know. The worst thing you could do is try to wing it. They never ask questions which they don't know the answer to already.
That said, confidence can go a long way, so try to relax and I'm sure the interview will go just fine. A simple trick I always used was to drink a beer an hour before the interview as it always seemed to calm my nerves and take the edge off. Good luck!