BCP 47 Language Tags — The Web Standard for Locale Identifiers

What Is BCP 47?

BCP 47 (Best Current Practice 47) is the IETF standard for identifying languages. It defines the syntax for language tags used across the web, in HTTP headers, HTML, JavaScript's Intl API, and almost every internationalization framework.

Tag Structure

A BCP 47 tag is composed of subtags separated by hyphens:

language[-script][-region][-variant][-extension]

Subtag	Length	Example	Description
Language	2-3 chars	en, zh	ISO 639-1 or 639-2 code
Script	4 chars	Hans, Latn	ISO 15924 script code
Region	2 chars or 3 digits	US, 419	ISO 3166-1 or UN M.49
Variant	5-8 chars	valencia	Dialect or spelling variant

Real-World Examples

Tag	Meaning
en	English (any region)
en-US	English as used in the United States
en-GB	English as used in Great Britain
zh-Hans	Chinese written in Simplified script
zh-Hant-TW	Chinese in Traditional script, Taiwan
sr-Latn-RS	Serbian in Latin script, Serbia
pt-BR	Portuguese as used in Brazil
es-419	Spanish for Latin America (UN M.49 region)

Matching Rules

BCP 47 defines matching algorithms:

1. Basic Filtering — prefix matching (en matches en-US)
2. Extended Filtering — wildcard subtags
3. Lookup — finds best single match by truncating subtags

The IANA Subtag Registry

All valid subtags are listed in the IANA Language Subtag Registry. This is the authoritative source for language, script, region, and variant subtags.