Hey Sander,

In a recent project doing something similar I took the approach of having a custom controller for my XML site map.

In the action I use this line to get all the domains for my root node (which has all the language elements)

var domains = Services.DomainService.GetAssignedDomains(rootItem.Id, false).ToList();

Then I call this recursive method:

private void AddItemToList(ICollection<SitemapUrl> urlList, IPublishedContent item, IList<IDomain> domains)
    {
        if(item.IsVisible() && item.TemplateId != 0 && item.DocumentTypeAlias.Equals("xmlSitemap") == false)
        {
            urlList.Add(new SitemapUrl
            {
                Url = item.UrlWithDomain().Trim('/'),
                LastModified = item.UpdateDate.ToString("yyyy-MM-dd"),
                Alternatives = domains?.Select(d => new SitemapUrlAlternative
                    {
                        Language = d.LanguageIsoCode,
                        Url = $"{Request.Url.Scheme}://{d.DomainName.TrimEnd('/')}{item.Url}".TrimEnd('/')
                    }).ToList()
            });
        }

        foreach(var child in item.Children())
        {
            AddItemToList(urlList, child, domains);
        }
    }

This works pretty well if you ask me :-)

Nik