I know that Germany has huge external debt. I don't know how to find out which entities own this and especially which country they belong to.
2 Answers
This is at the same time 1) a little old and 2) provides a deeper and more general answer that what you might be looking for, but I would still encourage you to go through it.
http://www.its.caltech.edu/~melliott/papers/financial_networks.pdf
with a preview in
https://www.youtube.com/watch?v=mjS81EoBQOU
(starting around 1'07)
and a complete didactic overview at
https://class.coursera.org/networksonline-003/lecture/211
and
https://class.coursera.org/networksonline-003/lecture/215
Among other things, you will learn that -- for most purposes -- it does not matter that much how much or in which country Germany (or any other country for that sake) has debts, but that the whole network or obligations is really what matters.