I came up with this, which seems to work, though it looks a bit hacky:
c::set('routes', array(
array(
'pattern' => '(:any)',
'action' => function($uri) {
if ($uri == 'de' or $uri == 'en' or substr( $uri, 0, 2 ) === "de" || substr($uri, 0, 2) == 'en') {
$site = kirby()->site();
$page = page('home');
return $site->visit($page);
} else {
$site = kirby()->site();
$page = page($uri);
go('en/' . $page);
}
}
)
));