Once you have the custom error page set up, you can add a code behind to it(from title and properties) or a simple user control which will perform a check for the culture of the browser currently accessing the page. After that, depending on the culture detected, a redirect to the error page in the appropriate language will be performed. For example, if it detects that the user is from Germany it will redirect him to the German page www.mysite.com/de/error.
I am glad that you found a solution. Should you have further questions, do not hesitate to open a support ticket.