Aleksander was born in Croatia and moved later with my family to Berlin, where he went to school and spent a big part of my childhood: Ja, ich bin ein Berliner, just like J.F.K. :) So German is for him both a foreign language and a mother tongue. This duality allows him to fully understand the basic problems my students might have with the language, but at the same time he can get into details with more complex stuff, just like a native would.