Today I fixed an issue in our open source plugin for EPiServer 7.5, Geta.SEO.Sitemaps, a tool to generate Google XML sitemaps. The issue occurred when you had one site mapped to multiple hosts with different language branches. Only the sitemap for the main SiteUrl worked (although displaying links to all language branches which is not desired) and the other hosts resulted in a 404.
The fix includes a modified admin UI and now it's possible to create sitemaps for all defined hosts of a site. All host names should show up in a dropdown list on the admin page when you add a new sitemap.
Given you have this configuration for your site:
All you have to do is create a sitemap for all the defined host names and you should be fine:
Sample output from english sitemap:
Sample output from norwegian sitemap:
Sample output from swedish sitemap: