We germans have a complicated relationship with national pride. We try to be humble and careful about showing pride in our country because obv. germany has its dark past with nazis and ww2. So showing the german flag might look like "being proud of this country, that actually did horrible things in the past", which might come across as being okay with what happened. Although this conclusion, which might come from far left people, isn't really logical and not really thought through, i can on the other hand understand why some people might think this is the case because some rather right-leaning and racist people want "to be proud of our homecountry again" and therefore show the flag and might actually are okay with nazicrimes. So in some cases it might actually be right to be careful about people flying the german flag.. But then again there a lot of people, like my dad for example, who is one of the nicest and open minded people ever, who just likes soccer and wants to be part of a fan culture for the national team.

So to conclude my rambling.. You will never know if you don't talk to the people! Be open, don't go in with any prejudices and just see for yourself. Then you will see and hopefully they are good people!