The Romans weren't the only foreigners to come to Germany. Today a lot of people from all over live there. Some tracks, past and present, of international residents:
Russian. There have been several waves of Russians. (Actually, there have been German settlements in Russia as well. It goes both ways.) One group of Russian settlers came with the army that was trying to chase Napoleon back to France. A group of them got tired of traveling and parked themselves in Potsdam just outside Berlin. You can see their traditional wooden houses and Russian Orthodox church here.
Then there were the Soviets after WWII. They've left their mark on Berlin and the former DDR as well.
After WWII, Germany was basically flattened, and so they invited foreign guest workers to help rebuild. Today there are something like four million Turks living there. The Turkish influence is a way of life--the most popular fast food in Germany is the Döner kebap.
Another large presence in Germany after WWII was...the Americans. Today the many military bases are due to joint projects by the two countries, not occupying forces. Here is the Stimson Memorial Chapel in Plittersdorf (a part of Bonn), which, as you can see, looks totally New England.
There are other groups, too. Germany is pretty welcoming of peaceful demonstrations, so a lot of people do them. Well, we did live there when a not-so-peaceful Kurdish demonstration happened, involving cobblestones flying through the air... But this is a peaceful hunger strike.
People come to German for all different reasons, and during a time like the World Cup, you can always see people showing their colors. You can see Sweden (yellow cross on blue) and Croatia (red, white, blue with checkered shield) here, alongside the German flags: