Hello and welcome to 4images forum.
Login as administrator (use same username and password as you've entered during installation process), go to ACP (Admin Control Panel), then click on "Einstellungen" (menu on left side), then select "English" in "Ordner der Sprachdateien wählen" dropdown, save the settings by clicking on "Änderungen speichern" button at the bottom.
P.S.
Yes, it's little confusing and should be changed, but on installation page you could choose the language