Translations of the Greenfoot user interface are included with the standard Greenfoot download.
Since Greenfoot 2.3.0, you can select the interface language from the preferences dialog within Greenfoot (under the "interface" tab).
At the link which I quoted above, there is even a german translated version of the API documentation (javadoc).
Live long and prosper,
Spock47
I want to clarify that I posted my response above only minutes after the question was asked (some hours before Super_Hippo's response). However, the post got held back due to checking it for spam (seems that a link to the greenfoot website triggers the spam alarm).
That's why there is now an additional answer shown despite Super_Hippo's answer already being completely sufficient.
Live long and prosper,
Spock47