`hreflang.code`

Default: error
Plugin: hreflang

An hreflang value is a valid language code, or x-default.

Why: Search engines ignore alternate links with an invalid code, such as english. A code is an ISO 639-1 language, with an optional ISO 15924 script and ISO 3166-1 region, such as en, zh-Hant, or pt-BR. Google does not support other codes, such as the reserved region UK (use GB), or the numeric region in es-419.

When off: any hreflang value passes.

<!-- ✗ Fails -->
<link rel="alternate" hreflang="english" href="https://example.com/en/" />
<link rel="alternate" hreflang="en-UK" href="https://example.com/uk/" />

<!-- ✓ Passes -->
<link rel="alternate" hreflang="en" href="https://example.com/en/" />
<link rel="alternate" hreflang="en-GB" href="https://example.com/uk/" />

References

Google: Localized versions of your pages: Google supports ISO 639-1 languages, ISO 15924 scripts, and ISO 3166-1 regions, and not codes such as UK or es-419.
Library of Congress: ISO 639 language codes: The ISO 639-1 language codes.
Unicode: ISO 15924 script codes: The ISO 15924 script codes.