Hi rasarab!
Hallo, Herr Reisetiger!
In the current version (1.0.1) there is a bug in line 83 of wp-downgrade.php which reads
if ($sprache == 'en_US'){
This bug affects anyone using a non-localized version of WordPress and prevents WP from downloading the correct US English WP version.
You should correct line 83 to instead read
if ($sprache == 'en_US/'){
(note the added slash after en_US) and you should be fine.
This fix is included in the unofficial version 1.1.0 I put together which is in English (und natürlich Deutsch).
Hope this helps.