Astro 5's i18n routing handles URL generation and locale detection, but it doesn't emit <link rel="alternate" hreflang> for you.
The naive fix — emit one alternate per supported locale on every page — quietly breaks when an entry only exists in one language. The alternate points at a 404, Google flags the reciprocal mismatch, and the hreflang is ignored. Worst case, it weakens the site's overall multilingual signal.
This post walks through the minimum override: walk Content Collections, detect which languages have the entry, then emit <link rel="alternate"> conditionally from Seo.astro.
What the override looks like
Three pieces:
