Hreflang Not Necessary For Non-Canonical Pages
It isn’t necessary to implement hreflang on the non-canonical version of a page because it won’t be shown in search, therefore any hreflang annotation won’t be used by Google.
Google Can Process Hreflang Tags Regardless of How They’re Implemented
It doesn’t matter to Google whether you implement hreflang tags in the HTML, in the HTTP header, in an XML sitemap or through a combination of these methods. Google will process the information in these tags regardless of the implementation method.
Hreflang Not Needed For All Page Types as Can Add Unnecessary Complexity
Use hreflang for pages which need to be linked between different language and country versions. Hreflang isn’t needed for all page types, like pagination and filtering, as this can add unnecessary complexity.
Use Hreflang & Canonical Tags to Handle Partially Translated Websites
John recommends using canonical tags and hreflang tags to tell Google which language version is preferred if you only have parts of a website that are translated, instead of the entire site.
Hreflang is a Minor Canonicalization Signal
Google uses hreflang as a small signal when picking the canonical version of a page, but this needs to be backed up by consistent signals provided by rel canonicals, internal linking and sitemap files.
Google May Fold Together Similar or Duplicate Hreflang Versions
Google may fold together hreflang versions of a page if the content is similar or the same, as it doesn’t make sense for both versions to be indexed.
Implement Clear Canonical Version Per Country & Hreflang Between Equivalent Canonical Versions
Don’t specify a canonical between different country versions of a page, because Google will likely only index the preferred version. John recommends having a clear canonical version per country and implementing hreflang between the canonical versions for each country.
After Indexing Google Checks to See if a More Relevant Page Can Be Swapped in
After pages are added into Google’s index, it then checks if there are any different URLs that should be swapped in because they are a better fit for the user.
Hreflang Alternates Can Still be Shown in Search While Technically Non-indexable Duplicates
Pages identified as duplicates which are not technically indexed may still be shown in search if they are an alternate hreflang version which is similar to the canonical version.