The longest one in Dutch I can think of is 'Hottentottententententoonstellingen"
Displays of tents of the Hottentots, or Khoikhoi as they are also called. The Hottentots lived somewhere in current West-Kaap, South-Africa.
EDIT: Just googled a bit, apparantly my language is kind of messed up this way, but we can just keep on adding if we want to, so: 'Hottentottententententoonstellingsmakersopleidingsprogramma'
Program to train the creators of displays of tents of the Hottentots
Or: 'Hottentottententententoonstellingsmakersopleidingsprogrammaverantwoordelijke'
The one responsible of the program to train the creators of tents of the Hottentots
Or, just to be silly: 'Hottentottententententoonstellingsmakersopleidingsprogrammaverantwoordelijkeopleidingsverantwoordlijke'
The one responsible of the program to train the one responsible of the program to train the creators of displays of tents of the Hottentots.
Also, I'm currently studying Chemistry and we have to be able to correctly create such words as those above (only shorter, though) if given a structural drawing. It can be a pain...