The gitub upstream repository contains many updated and new
translations (po files). It would be great if you could pull those
updated / new po files into the version distributed by Debian. All
files are hosted in data/po in the (upstream) repository.

Please note that these files include a few typo fixes. So additionally
it would be very helpful if you could also update the following
files:
src/editor/editor_screen.cpp    // contains further changes, so possibly not
data/levels/desert/desert5-tflavel.pingus    // contains further changes, so possibly not
data/levels/mysteryisland/madscientist1-lac.pingus
data/levels/mysteryisland/rainy1-lac.pingus
data/levels/factorycampaign/factory_campaign1.pingus
data/levels/xmas2011/xmas06-grumbel.pingus
data/levels/xmas2011/xmas09-grumbel.pingus

For the first two I can provide you with minimal patches, if you want.

Note that solving this bug report supersedes the patches
more-updated-German-translation.patch
update_german_translation.patch

If you want, I can prepare appropriate patches for debian/patches.

If there are open questions while solving this, do not hesitate to
ask.