This document describes mechanisms for identifying or selecting the language of content or locale preferences used to process information using Web technologies. It describes how document formats, specifications, and implementations should handle the language tags described by .

This specification will not deal with formats for locale data or actual locale data. One possible source of locale data and data formats is .

Identification of language and locale has a broad range of applications within the World Wide Web. Existing standards which make use of language identification includes the xml:lang attribute in , the lang and hreflang attributes in , or the language property in . Locale identification is used for example within the CLDR project, cf. .

This specification defines normative guidelines and Best Practices in four areas:

This document uses the terms language tag and subtag which are defined in or its successor.

In addition, this document uses the following terms, which are defined in or its successor:

For examples, see .

BEST PRACTICE: Use language as the core of locale identifiers.

"Locale" is a fairly old concept coming from the field of software localization in the 1980's. Localization is understood to mean doing whatever it takes to adapt a piece of software to a given group of users; we're talking about large groups here, such as a whole country or all the speakers of a certain language. The "locale", then, is the set of "things" common to this group, from the point of view of the software being localized. The most important part of localization is the translation of all text to the language of the users, so that they can understand it. But there are other aspects:

BEST PRACTICE: If possible, protocols should have the same field for conveying language and locale information, except in cases where the notion of locale encompasses extended information (see ).

Historically, natural language identifiers have been used as locale identifiers by some programming languages or operating environments, which is natural since locale identifiers usually share certain core features related to natural language and country/region.

A major difference between language tags and locale identifiers is the meaning of the region code. In both language tags and locales, the region code indicates variation in language (as with regional dialects) or presentation and format (such as number or date formats). In a locale, the region code is also sometimes used to indicate the physical location, market, legal, or other governing policies for the user.

The language tag may be available in several places. In HTTP, there is an Accept-Language header field which can be used. MIME has a Content-Language header which contains a language tag. In XML, there is an attribute which can be defined for elements called xml:lang. xml:lang marks all the contents and attribute values of the corresponding element as belonging to the language identified. What that means for processing those contents varies from application to application.

For more detailed information on the behavior of xml:lang, see .

BEST PRACTICE: Use for language identifiers and as the basis for locale identification.

A minimal requirement for locale identifiers is the ability to specify the natural language; thus there is industry convergence on the use of or its successor as the core of a locale identifier. For example, uses or its successor as the core of a locale identifier, and provides syntax for extensions for non-linguistic information, such as preferred currency or timezone.

or its successor refer to language identification only. Locales can be identified in several ways. One method is by inference from language tags. For example, an implementation could map a language tag from an existing protocol, such as HTTP's Accept-Language header, to its locale model. Locales may also be identified directly by using the language tag syntax in data items (elements, attributes, headers, etc.) that explicitly serve the purpose of locale identification.

BEST PRACTICE: For locale identification, create variants of identifiers which can be canonicalized.

BEST PRACTICE: Avoid if possible the usage of legacy formats.