Hello Paul Markham,
This is a problem with the localization and path prefix. You see that ASP.NET treats the /de path as another folder thus it looks for the default document there and Sitefinity loads your home page. You can try creating a folder /de in your website root and see what will be the result.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items