install Market Ready Germany module
http://www.panticz.de/magento_market-ready-germany

Edit condition
Admin Panel > CMS > Static Blocks
click on "AGB"
edit Content
[ Save Block ]

click on "Widerrufsbelehrung"
edit Content
[ Save Block ]

#### OLD ###

# ToDo
# create Terms and Conditions for EVERY language in shop

Create condition
Admin Panel > Sales > Terms and Conditions
[ Add New Condition ]
Terms and Conditions Information
Condition Name: AGBs
Status: Enabled
Checkbox text: Ich habe die Allgemeinen Geschäftsbedingungen (AGB) gelesen und akzeptiere diese
Content: PASTE_YOUR_CONDITIONS_HERE
[ Save Condition ]

Enable conditions on checkout
Admin Panel
System > Configuration > Sales > Checkout
Checkout Options > Enable Terms and Conditions > Yes
Save Config