Usually you need to refresh the repository's listings.
Code:
sudo apt-get update
sudo apt-get install libreoffice libreoffice-l10n-de libreoffice-help-de
If that is not working then look at what is or isn't in the repostiory and the repository itself:
Code:
apt-cache policy libreoffice
apt-cache policy
Also, if you are already logged in as root, it is not necessary to use
sudo to become root again.
Edit: if the above does not work, then also try this before installing:
It may ask a question or two about the repositories.