Prepare as you would for any job interview.
Be confident be yourself
Be very professional and prepared.
You should be fully concentrated on your technical knowledge, be familiar with the Main company to some extent in detail, knowing German could be a plus, You should be totally honest.
but try not to be modest, do not underestimate yourself, a good presentation is a key, self-confidence is a key.
I did not enter in a interview at Siemens. Because in my ex company which I mentioned it before as Guncan Energy. We were buying engines and turbines from Siemens AG. So they accept me.