Hugo version: 0.30
0859d9dfe647db3b8a192da38ad7efb5480a29a1 wraps xmlns:xhtml="http://www.w3.org/1999/xhtml" with if .IsTranslated block.
But as far as I know, .IsTranslated will always return false for the sitemap.xml template. So, xmlns:xhtml="http://www.w3.org/1999/xhtml" is never rendered. And it causes the following error:
Namespace prefix xhtml on link is not defined
I might be wrong, but I think #3933 should be reverted.
I've tested this on a multilingual site.
Where do you get that validation error? Is it a "blocker"?
With the mentioned error present, Google Webmasters tool reports that the sitemap.xml file can't be parsed:
Google Webmasters tool

Google Chrome browser

OK, that's not good.
Answer to https://github.com/gohugoio/hugo/issues/3978#issuecomment-337248361
.. Is it a "blocker"?
Yes, this prevents me to update from 0.27.1 to 0.3x