Hi Jan,
> it's a too big a job to do by myself, and maybe the answer already exists:
>
> When I would download all Debian source packages, extract them, determine
> of each the programming language it is written in and the SLOC, what would be
> the percentages of each programming language used?
>
[...]
While zack already provided you with much more detailed instructions to
obtaining up-to-date information, you might find
http://blog.james.rcpt.to/2012/02/13/debian-wheezy-us19-billion-your-price-free/
useful to get an overview without extra effort.
Best,
Michael