About the Hreflang Checker
The Hreflang Checker extracts every language annotation from a page and validates the set: unique language codes, a self-reference, an x-default fallback and https alternates. Broken hreflang means Google shows the wrong language version in the wrong country — exactly what multilingual setups like a DE/EN/FR/ES site need to verify. Remember to run the target pages too: hreflang only counts when it is reciprocal.
x-default names the page shown to users whose language matches none of your variants (often the language selector or the English version). Google recommends it for every hreflang set.
Page A pointing to B only counts when B points back to A. Missing return links are the number-one reason hreflang is ignored — check both directions.
ISO 639-1 language (de, en) optionally plus ISO 3166-1 region (de-AT, en-US) — region alone (like "eu") is invalid, and codes like "uk" mean Ukrainian, not United Kingdom.