hreflang
The tag that tells search engines 'this page also exists in French and German', so they can point each searcher at the right version.
See it
What it is
hreflang is a small annotation (a link tag in the head, an HTTP header, or an entry in your XML sitemap) that says 'this page has these other language and region versions, here are their URLs'. Search engines treat it as a targeting hint when they pick which of several equivalent locale URLs to show a given searcher. A hint, not a promise: it does not decide what any one visitor sees, and it is not the fix for duplicate-content handling, which runs through canonicalization instead.
Reach for it the moment you ship a second locale at a second URL, whether that is /es/, es.example.com, or example.es. Values are BCP 47 tags: 'en', 'en-GB', 'pt-BR', plus the special 'x-default' for the page you serve when nothing matches. Every version in the set must list every other version, itself included, and each of those pages must link back. One-way hreflang is ignored.
Two traps. First, the region half is a country, not a language: 'en-UK' is invalid, the tag is 'en-GB'. Second, hreflang is a hint about which version to show, not a ranking boost and not a canonical. If your canonical tags point every locale at the English page, hreflang loses and your translations stay unindexed.
Ask AI for it
Add hreflang annotations to this multilingual site. For every page, emit a complete set of <link rel='alternate' hreflang='...' href='...'> tags in the head covering all locale variants, including a self-referencing tag and an x-default pointing at the language-selector or English page. Use valid BCP 47 tags (en-GB, pt-BR, es-MX, not en-UK), absolute URLs, and make the set fully reciprocal so each variant lists every other variant. Ensure each page's canonical points at itself, not at the source-language page.