RewriteEngine On

RewriteCond %{HTTP:Accept-Language} ^en [NC]
RewriteRule ^$ http://siteadi.com/en/ [L,R=301]

RewriteCond %{HTTP:Accept-Language} ^de [NC]
RewriteRule ^$ http://siteadi.com/de/ [L,R=301]

RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-l

RewriteRule ^(en|de)/?$ index.php?lang=$1 [QSA,NC,L]

şu şekilde denermisin