I was just testing the UrlRewriteExtension functionality and I instantly encountered a problem. I made a new installation of version 5.1.422.4 along with the globalized template package and sample content. The link to the start page (clicking on the logo or the "Start" link in the main menu) results in a 404 error when a request is made to e.g. http://localhost/en.html.
Hello.
Unfortunately this is a bug. Your bug reference number is 7393 for future reference if contacting support.
Thank you for bringing this to our attention.