Translated content not accessible / wrong links
In addition to enabling "URL" Detection method (path prefix or domain) and positioning it to be used first, you also have to Configure it. 


Set German as your Default language under  Administration > Configuration > Regional and language 
Go to the Language Detection and Selection page and enable the "URL" Detection method
On the Language Detection and Selection page click through the Configure button for the URL (/admin/config/regional/language/detection/url)
Select "Path prefix" as the " Part of the URL that determines language" option
Delete the de from the "German (de) path prefix (Default language)" option
Clear cache! This can be tricky through the interface if you're using the admin module because the /de/ might linger on the menus but will lead to a Page not found. Delete the /de/ in your browser Location bar from the flush cache link to make it work. 


After this you should have the correct Home in German, without the /de, German content and links without the prefix and English content and links with /en/.