Thanks for pointing it out. I just fixed my copy, so it will be fixed in the next release.
The problem was that the pagenotfound page is not accessed directly. The pagenotfoundhandler makes a web request to get the contents of the page then it renders the content and sets the response status to 404. So the webrequest made by the server is not using the browser language preference of the user. I fixed it to pass a culture parameter bawsed on the user's browser language.