Your best bet is to add a custom fields in your Page class, such as GermanTitle and GermanContent
Then make 2 functions, Title() and Content(), on your Page class:
function Title() {
if( /* current language is german */) {
return $this->GermanTitle;
} else {
return $this->Title;
}
}
The final thing you've got to choose is how to switch between German and English. The simplest is probably a session variable, although this isn't very Google-friendly.
An alternative would be to set up a Director rule (call to Director::addRules) that mapped de/$URLSegment/$Action to a German version of ModelAsController.
I haven't spelled it out completely, but this should get you started. Good luck!