The plugin should output rel="alternate" hreflang="x" header for the current locale, as well as ones for translations.
Ref: https://support.google.com/webmasters/answer/189077

git-svn-id: https://plugins.svn.wordpress.org/bogo/trunk@925099 b8457f37-d9ea-0310-8a92-e5e31aec5664