I changed the line "define('WPLANG', 'de_DE');" to "define('WPLANG', '');" at my multisite instalation. I did it with a file editor plugin in the admin area. After that the site broke and I become the error: "HTTP-Fehler 500 (Internal Server Error):"
I reversed the changes in the wp-config.php but the site is still broken.
Do you have any idea how can I resolve this?
Thank you everyone!