Depending on what gateway you are working with the checkout_success should either be listed in the local checkout server behavior or if it is a remote checkout it will be listed as the value of an element in the form on your confirm page.
One thing that you could do is to set a session variable to hold the value of the checkout success page. When you set the language session variable you should also set another variable to store the checkout success page that the user will need to go to. Then on your confirm page you can just reference this checkout success session variable rather than the page name directly. This way it should always go to the correct success page based on the language the user selected.
