Build hreflang annotations for multilingual sites, validated as a set.
Optional — checks that it appears among the alternates
hreflang must be reciprocal — this same block belongs on every page it references, each listing all the others including itself. A one-way annotation is discarded entirely, which is the most common reason hreflang appears to do nothing.
Two languages
The minimum useful set
en, frLanguage and region
Uppercase region subtag
en-GB, en-USWith a default
For unmatched visitors
x-defaultA language code and the absolute URL for that version.
The fallback shown when no language matches the visitor.
The set must appear on all pages it references, or it is ignored.
Duplicate codes, missing reciprocity and a canonical outside the set are group-level problems that per-tag checking cannot see.
"en-gb" is invalid where "en-GB" is correct — the language is lowercase and the region uppercase, a distinction that silently voids the annotation.
The most common hreflang failure is a one-way annotation, so the requirement is repeated on every result rather than buried in documentation.
Generate the rel="alternate" hreflang tags that tell search engines which language and region each version of a page serves. Hreflang is validated as a group rather than tag by tag, because that is how the rules work: codes must be valid ISO values with an uppercase region subtag, each code may appear only once, every URL must be absolute, and the annotation must be reciprocal — every page in the set listing every other, including itself. A one-way annotation is discarded entirely, which is why that requirement is stated on every result.
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