The biggest takeaway I got from How to Win Friends and Influence People is simply this: Think about things from the other person's perspective. Instead of talking about yourself, let them talk about themself. Instead of boasting about your own accomplishments, ask about theirs.
The exact way you do these things depend on the culture for sure, and the book is written by an American. From what I understand Germany is less directly "friendly" than the US so the pleasantries may not carry over, but I have to imagine that considering the other person more than yourself does carry over.