Is is possible to set up IIS (or a load balancer of some kind) so that each URL of the form...
http://www.domain.com/en/
http://www.domain.com/fr/
http://www.domain.com/es/
etc...
... is handled by a different server? There should be no HTTP redirection, i.e. the URL stays as it is in the user's browser.
Yes, you can tell IIS to proxy requests for particular URL subsets to other servers using the URL Rewrite module and Application Request Routing module. Some additional documentation is available at http://learn.iis.net/page.aspx/740/application-request-routing-module/