Use this when
hreflang, i18n SEO, international SEO, multi-language, multi-region.
Process
Validate every page in the set against these rules: a self-referencing tag on every page pointing to itself, matching its canonical exactly (missing this causes Google to ignore the whole hreflang set); full bidirectional return tags (if A links to B, B must link back to A — check the full mesh, not just spot pairs); at most one x-default per set, itself with return tags from every other version; ISO 639-1 two-letter language codes with an optional ISO 15924 script subtag when needed for script-specific targeting (zh-Hans/zh-Hant), never ISO 639-2 codes like eng; ISO 3166-1 Alpha-2 region codes only when paired with a language (a bare region code like be is actually the Belarusian language code, not Belgium — a country code alone is invalid); hreflang only on canonical URLs, never on a page whose own canonical points elsewhere; consistent protocol (all HTTPS) across the whole set.
Note plainly that hreflang is a hint, not a directive, and that Google ignores locational meta tags and HTML geotargeting attributes entirely — ccTLD, hreflang, and server location are the only real geo-signals, in roughly that order of influence, and the Search Console International Targeting report was removed in 2022 so there's no manual country-targeting lever left to recommend.
Generate implementations in whichever of the three methods fits: HTML <link> tags for smaller sites (under 50 variants per page), HTTP headers for non-HTML files like PDFs, or an XML sitemap with hreflang entries for larger or cross-domain sites (recommended once the set gets big, since every <url> entry needs the complete set of alternates including itself).
Cultural adaptation pass, once the technical implementation is sound: check whether CTAs match cultural expectations (direct/imperative for the US, more indirect/informational for DACH and Japan), whether trust signals are locale-appropriate, whether foreign brand references leaked into a localized page, and whether numbers/dates/currency formatting matches the target locale. Flag cultural-adaptation issues as Medium severity — real, but distinct from a technical hreflang break.
Content parity audit across language versions when asked: page existence across every declared language, section-structure equivalence, SEO-element parity (title/meta/schema localized, not just carried over), a word-count ratio sanity check (German commonly runs 25-35% longer than English, Japanese 10-25% shorter), and staleness detection via timestamps.
Report a validation results table (language, URL, self-ref, return tags, x-default, status), generated tags/sitemap/headers as needed, and — when requested — the cultural adaptation score and content-parity matrix.