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 , as well as data structures that extend these tags to describe international preferences (see sec. 3.1 in ).

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 atttributes in , or the language property in . Locale identification is used for example within the CLDR project, cf. .

The current best practice when developing specifications for language identification is to refer to , using a formulation like RFC 3066 or its successor. Recently a successor for has been developed, called . This specification takes as the basis for language identification, and as the basis for matching of language identifiers ("tags").

The current practice in many standards is to identify language in terms of , using formulations like RFC 3066 or its successor. Recently a successor for has been developed, called . This specification takes as the basis for language identification, and as the basis for matching of language tags.

refers 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.

Currently, this specification refers to and directly. Since and are expected to become the new BCP 47 before this working draft becomes a recommendation, a later draft of this specification will refer to BCP 47 directly.

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

In order to enable multi-locale operation of Web services and to create the ability for locale negotiation, this specification describes a standardized method for identifying locales and locale and/or language tags on the Web, including non-normative guidelines for implementation. This is called out in Requirement R005 of . The mechanism for language and locale identification which is defined in this specification will be used in a future version of the description of Web services Internationalization in .

Further application scenarios of this specification encompass for example the standards mentioned in . The scenarios can be divided in four areas:

As for matching of language tags, many specifications already define operations using matching. An example is the language pseudo-class :lang defined in sec. 5.11.4 of . It matches elements based on their language. This specification formulates requirements on such operations, based on .

This document defines locale identifiers for use in Web technologies. 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. This specification defines locale identifiers that specific locale implementations can map to their proprietary features in order to create functional, interoperable applications.

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

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 .

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

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