1. Introduction

   Human beings on our planet have, past and present, used a number of
   languages.  There are many reasons why one would want to identify the
   language used when presenting or requesting information.

   A user's language preferences often need to be identified so that
   appropriate processing can be applied.  For example, the user's
   language preferences in a Web browser can be used to select Web pages
   appropriately.  Language preferences can also be used to select among
   tools (such as dictionaries) to assist in the processing or
   understanding of content in different languages.

   In addition, knowledge about the particular language used by some
   piece of information content might be useful or even required by some
   types of processing; for example, spell-checking, computer-
   synthesized speech, Braille transcription, or high-quality print
   renderings.

   One means of indicating the language used is by labeling the
   information content with an identifier or "tag".  These tags can be
   used to specify user preferences when selecting information content,
   or for labeling additional attributes of content and associated
   resources.

   Tags can also be used to indicate additional language attributes of
   content.  For example, indicating specific information about the
   dialect, writing system, or orthography used in a document or
   resource may enable the user to obtain information in a form that
   they can understand, or it can be important in processing or
   rendering the given content into an appropriate form or style.

   This document specifies a particular identifier mechanism (the
   language tag) and a registration function for values to be used to
   form tags.  It also defines a mechanism for private use values and
   future extension.

   This document, in combination with [RFC4647], replaces [RFC3066],
   which replaced [RFC1766].  For a list of changes in this document,
   see Section 8.

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].