The Russian characters in series 3 speak Russian with Balkan accents.
The Kosovan Albanian character Irena is played by a Serbian actress, Zelda Tinska. This is ironic given the conflict between the Albanian and Serbian characters in the series.
Most of the German location shots in Series 1 were filmed in Hamburg, not Dusseldorf. One of the giveaways is that many of the cars have HH (Hamburg) number plates.