For SEO purposes, the sitemap.xml should contain "X-DEFAULT" links in the translation sets.
These links tell Google (and other search engines) which translation to show if the user's language is not included in the translation set.
For more info see here:
At the moment we don't have a way to add X-Default link.
Update the default generator to provide the X-Default link.
The link default language should be the same as the default language of the site or content.
Add a new Add X-Default (x_default) language setting.
Update config schema (add a new x_default setting) and provide an upgrade path.
Active
4.0
Code
Provided a solution for X-Default but tests still must be provided
Supplying patch to get #3 working with the latest 4.x codebase