> As already pointed out by Nicolas Goaziou, org-mode also requires other
 > external stuff

A data point.

Ditaa requires a JRE as mentioned at

  http://orgmode.org/worg/org-contrib/babel/languages/ob-doc-ditaa.html

The litigious terms are

  ditaa version 0.9, Copyright (C) 2004--2009  Efstathios (Stathis) Sideris

I didn’t find ditaa.jar in org-mode’s contrib/scripts as claimed.