ISO 639 language codes
Also called: iso 639, language codes, language code
ISO 639 is the international standard that assigns short codes to languages. ISO 639-1 gives each major language a two-letter code (en, de, fr, zh), and those alpha-2 codes are what Google's hreflang attribute uses to label a page's language.
From standard to hreflang tag
ISO 639 is a family of standards, not a single list. Part 1 (ISO 639-1) is the alpha-2 set: two lowercase letters per language, with 183 codes registered as of June 2021 and administered by Infoterm. Parts 2 and 3 use three-letter (alpha-3) codes to cover thousands more languages and dialects. For search, the two-letter 639-1 code is the one that matters.
Google’s hreflang annotation reads a 639-1 language code, optionally followed by an ISO 3166-1 Alpha 2 region code:
de: German, any regionen-GB: English for the United Kingdompt-BR: Portuguese for Brazil
Codes outside those two standards are ignored. es-419 (Latin America, which is a UN M49 region rather than ISO) does not work, a region value on its own is invalid, and UK fails because ISO uses GB for the United Kingdom. Language codes are case-insensitive but written lowercase by convention, and region codes uppercase. When no localized version fits a user, the reserved x-default value marks the fallback page.
A single wrong token (writing ge instead of de, or en-EU where no EU region exists) silently voids that annotation, so the return pairing between two pages never forms and Google treats the signal as if it were absent.
How it affects your traffic
International e-commerce stores lean on hreflang to route shoppers to the right storefront, and one malformed ISO 639 code can break the whole cluster. Tag a German catalog with an invalid language code, or reach for es-419 on a Latin American store, and Google drops the annotation, which can surface the wrong-currency or wrong-shipping page in results or let translated pages compete against each other as duplicates. Clean two-letter codes are among the cheapest fixes in international e-commerce SEO, which is where multi-region catalog work usually starts.
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