BCP47

BCP 47, Best Current Practice 47, is an IETF standard that defines a comprehensive system for identifying human languages in software, data formats, and communications. It provides a uniform way to tag language, script, region, and related variants so applications can select appropriate content, locales, and processing rules.

A language tag in BCP 47 is a hyphen-separated sequence of subtags. The primary language subtag comes from ISO 639, optionally followed by a script subtag from ISO 15924, and a region subtag from ISO 3166-1. Additional variant subtags may refine meaning, and extension subtags can add structured information using single-letter prefixes. There is also a private-use mechanism, indicated by an x- prefix, for user-defined subtags. Tags may include grandfathered or irregular forms that predate or diverge from the core rule set.

The official registry for subtags is maintained by IANA, in the Language Subtag Registry. Subtags are registered to enable consistent tagging across systems and standards. The standard also defines rules for canonicalization and matching, including case normalization (language subtags are generally lower-case, script subtags title-case, region subtags upper-case). This enables robust comparisons and graceful fallback when exact tags are not available.

BCP 47 is widely used in computing and web technologies. It underpins language selection in the HTML lang attribute, HTTP Accept-Language headers, and various internationalization libraries and frameworks. It supersedes earlier language-tag specifications and is maintained by the IETF, with ongoing updates to accommodate new languages, scripts, and regional distinctions.