Two nieces (16 & 18) who have never traveled internationally by themselves are planning a trip to Hamburg (from the US) to visit a friend. They don't speak German. I have discouraged them from connecting at the maze that's called Frankfurt. The cheapest airfare they can find, apparently, is via Duesseldorf on Air Berlin. It's been ages since I have flown into DUS, and I have no idea how easy it is to make a connection there to a domestic flight. Anyone?