Berlin is one of the most diverse cities in Germany. If you want to see districts that seem to look more German, those are relative be it if you're in Jannowitzbrücke ( I like that area esp. because of its famous song), Ostkreuz, Warschau Strasse, Alexanderplatz.
One of the reasons for a diverse mix of inhabitants is in numerous cities are universities and other institutions of higher learning, such as Potsdam, Kiel, Frankfurt an der Oder, etc where international students from all over are there.
The diversity in cuisine listed above: which people do you expect to see in these restaurants, Chinese, Italian, Greek, Turkish, Persian, Vietnamese, Mexican, etc, etc. The patrons are Germans filling up the Vietnamese restaurants and so on. Check out Savignyplatz, main streets in Charlottenburg, Alexanderplatz, Nikolaiviertel.
For a "traditional" German food restaurant, I suggest two places, one of which I've been to a few times, both lunch and dinner, located on the side street parallel to Rathaus Koepenick. This is "Restauration zur Gardestube" on Rosenstrasse. Patrons are only German, the times I was there, menu is in English and German, a good place to use your German. The food is Berlin style.
This 2nd traditional restaurant is in the Nikolaiviertel very close to Alexanderplatz, "Mutter Hoppe" It's crowded, lots of tourists and locals as well, be prepared to wait, not sure if the place takes reservations.
If you want Chinese cuisine, a super popular restaurant is on Kantstrasse in Charlottenburg, always crowded at dinner, packed with Germans, local tourists and other nationalities living in Berlin. It's "China Friends" The Savignyplatz area is very diverse in its cuisines, be French, Chinese, Vietnamese, Greek, German, Spanish, etc.