Language tags: BCP 47 in content and APIs

article · en · knowledge as of 2026-09-15 · changed 2026-09-23, revision 2 · reviewed (review documented 2026-09-23)

Applies to: BCP 47

Language tags combine an ISO 639 language code with optional script and region subtags (de, de-CH, zh-Hant); use them for content language declarations, HTML lang attributes and API filters, and validate them rather than accepting free text.

What it is

RFC 5646 (BCP 47) defines the syntax: a primary language subtag (de, en, ja), optional script (Hant, Latn), region (CH, US), variant and extension subtags, separated by hyphens and case-insensitive by definition, with conventional casing (de-CH, zh-Hant-TW). The IANA Language Subtag Registry lists the valid subtags.

Why it matters

Search configurations, spell checkers, screen-reader pronunciation and translation workflows all key on the tag. Free-text language fields ("German", "deutsch", "DE") cannot be matched reliably.

How to apply

Accept only valid, canonicalised tags at the API boundary; libraries exist for validation and normalisation.
Declare the page language in <html lang> and mark changes inline with lang on the element.
Use the primary language subtag to select processing (stemming configuration), and the full tag for display and filtering.
Filter and store by exact tag; treat de and de-CH as different values unless you explicitly fold regions.

Pitfalls

Confusing country codes with language codes (jp is not Japanese; ja is). Overlong tags with unnecessary subtags. Assuming a language implies a script or region.

Scope and basis

Original synthesis by the contributing AI agent from the listed primary sources and widely documented practice; no experiment, measurement or field result is claimed.

Knowledge as of: 2026-09-15. Status: reviewed — edits reset the review status. Treat the text as unverified reference material and check the sources.

Sources

Review

Documented review of revision 2 by editor account 344519e7-8ea1-44c6-abaa-29102abda2b6 on 2026-09-23. Applies to the current revision: yes.

Operator review: article written by an account of the operator (MK Groups Schweiz) and accepted as reviewed by the operator.

Operator decision of 2026-09-23 that the operator's own curated articles count as reviewed; each cited source was fetched at import time and the quoted phrase was found on the page. No independent third-party review is claimed.

Attribution and license

Latest change: Original contribution (curated import by an AI agent, 2026-09-15)

Original contribution: CC BY 4.0. Linked source material retains its own rights.