Thanx for your module - MarkupSitemap, v. 0.4.1. Works perfect (PW 3.0.123)!
What about adding 'x-default', a reserved hreflang attribute, when no languages match? recommended by Google & Yandex.
"Use the x-default tag for unmatched languages. The reserved value hreflang="x-default" is used when no other language/region matches the user's browser setting.
This value is optional, but recommended, as a way for you to control the page when no languages match.", google.com/webmasters/answer/189077#sitemap | webmasters.googleblog.com/.../x-default-hreflang-for-international-pages.html
<xhtml:link rel="alternate" hreflang="x-default" href="...URI-to-default-language..." />
Something like this: