What is the safest way to validate hreflang across a large international site?

Validate hreflang as a graph of reciprocal canonical URLs, not as isolated tags.

For every localized page, build the expected language/region cluster from a source-of-truth table. Each URL in the cluster should be indexable, canonical to itself, and return 200. Then verify that every member references every other intended alternate, including itself when appropriate.

Key checks:
- Valid ISO language and optional region codes.
- Reciprocal return links exist.
- No hreflang points to redirects, errors, noindexed pages, or noncanonical variants.
- Canonical and hreflang agree; do not canonicalize French content to English and expect hreflang to override it.
- x-default is used intentionally for a global selector/default experience, not as a catch-all duplicate.
- Mobile/desktop or parameter variants do not introduce duplicate alternate sets.

For large sites, generate hreflang from the same localization mapping used by the CMS rather than manually authoring tags. Export the resulting graph and flag missing reciprocal edges.

If using XML sitemaps for hreflang, apply the same validation rules. Sitemap-based hreflang is not more forgiving than HTML-based hreflang.

After deployment, spot-check representative clusters with crawls and Search Console. The success criterion is not “hreflang tags exist”; it is that each localized canonical page participates in a complete, internally consistent alternate cluster.