The Hreflang Checker reads the hreflang tags in a page's HTML and lists each language and URL. It checks for an x-default fallback, an entry for the page itself, valid language codes and duplicate codes. If the page has no hreflang tags, it says so and shows the page's html lang value. It does not check the return tags on the other pages.
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The headline gives the number of hreflang tags found, and the table lists each language code with its URL. Four checks sit above it.
If there are no tags, the result is informational. That is fine for a site in one language. If you have translated versions, add the tags so search engines can tell the versions apart.
x-default set warns when no entry uses x-default, the fallback for visitors whose language you do not cover. Self-referencing entry fails when the page does not list its own URL in the set. Each page in a set needs to include itself.
Valid language codes accepts a 2-letter language code with an optional 2-letter region, like en or en-GB, plus x-default. Anything else is flagged, including codes with a script such as zh-Hans, so check those by hand. No duplicate codes fails when the same code appears more than once.
Return tags are not checked. Every page you list must point back at this one, so open each target and run the check there too.
Peak Answer runs checks like these every week, then writes the content and gets the placements.