Unfortunately, that's not possible right now, but we will soon enable such option in the next updates.
if you need any help by translating the app (or single translation files) into German, I believe I can help (as native German).
Just contact me if interested!
Thank you very much for your offer. Our app is already fully translated into multiple languages incl. German and the new version which will support multi-lingual interface is already in the submission process.
Of course you can always help us with the translation at http://translate.todoist.com/