I think there's a loop in your redirection. The RewriteCond matches domain.de and then rewrites to domain.de.. thus it will always match and always redirect. You should just match the first level and rewrite www.domain.de to www.domain.de/de/ and nothing more?
Something like this should work and matches www.domain.de and domain.de
RewriteCond %{HTTP_HOST} ^(www\.)?domain\.de$ [NC]
RewriteRule ^/?$ http://www.domain.de/de/ [R=301,NC,L]