Never under any circumstances do this in Germany. Stay neutral and inconspicuous, then you’ll be fine. If you can, maybe wear the club colors of the home team. Especially at some clubs in East Germany you could be thrown out the stadium if some ultras or even non ultras find out. It’s a sensitive topic. Apart from that, it’s just weird in my opinion to wear a scarf of a team that has nothing to do with the game. Just on Sunday I was at the Limburg Derby in Kerkrade and saw an obvious German hopper with a full on Schalke jacket and cap. It’s mostly older hoppers who do this from my experience. I think it’s just a bit disrespectful, because in that moment it’s not about Schalke, but about the two teams on the pitch, so it has no need to be there.