As I thought the error was simply a couple of formatting errors in the German language translation file.
The fix just needs one file replaced, which is attached here. Download the zip folder it to your computer, then extract the file it contains (“de.mo”) to your computer,. Then upload that file to the kiwitrees installation on your web server. Put it in the folder “kiwitrees/languages/”, replacing the file of the same name that is already there.
If it doesn’t immediately solve the problem navigate to the page Administration > Site administration > Clean up data folder; select the folder “cache”; and delete it. This resets all the cached translations, which will then be immediately re-cached, but including the new German translation.