This morning, Google began sending thousands, if not, hundreds of thousands, of notifications to webmasters who have potentially implemented hreflang incorrectly on their websites.
Here is a picture of a notification Google sent to Joost de Valk from Yoast:
Google had detected that some pages on your site have implemented the rel-alternate-hreflang tag incorrectly. In particular, there seems to be a problem with incorrrect language and region codes, or incorrect bi-directional linking (if page A links with hreflang to page B, there must be a link back from B to A as well). Google uses the hreflang attributes to serve the correct language or regional URL to the right users in search results.
Until you correct the errors on the hreflang links, your website will not benefit from additional language or region targeting.
Google’s Gary Illyes confirmed with me on Twitter that these are a brand new type of notification. He would give the specific number of notifications being sent, but if I had to guess, it would be in the hundreds of thousands. Gary said it was about 15% of the notifications sent out for the mobile usability issues found on web sites and it seemed like every webmaster I know received that notification.
If you get such a notification, that means you need to resolve the issues in order to benefit from the “additional language or region targeting” offered by hreflang.
I suspect Google found some common implementation errors and was able to mass notify users with such issues.