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SECTION 10.8                                                    Accessibility Support



fied by a Lang entry in the document catalog (see Section 3.6.1, “Document Cat-
alog”). Below this, the language can be specified for the following items:
• Structure elements of any type (see Section 10.6.1, “Structure Hierarchy”),
  through a Lang entry in the structure element dictionary.

• Marked-content sequences that are not in the structure hierarchy (see Section
  10.5, “Marked Content”), through a Lang entry in a property list attached to the
  marked-content sequence with a Span tag. (Although Span is also a standard
  structure type, as described under “Inline-Level Structure Elements” on page
  905, its use here is entirely independent of logical structure.)

The natural language used for optional content allows content to be hidden or re-
vealed, based on the Lang entry (PDF 1.5) in the Language dictionary of an op-
tional content usage dictionary.

The following sections provide details on the value of the Lang entry and the
hierarchical manner in which the language for text in a document is determined.

Note: Text strings encoded in Unicode may include an escape sequence or language
tag indicating the language of the text and overriding the prevailing Lang entry (see
Section , “Text String Type”).

Language Identifiers

Certain language-related dictionary entries are text strings that specify language
identifiers. Such text strings appear as Lang entries in the following structures or
dictionaries:
• Document catalog, structure element dictionary, or property list
• Optional content usage dictionary’s Language dictionary, although the hierar-
  chical issues described in “Language Specification Hierarchy,” below do not ap-
  ply to this entry

A language identifier can either be the empty text string, to indicate that the lan-
guage is unknown, or a Language-Tag as defined in RFC 3066, Tags for the Identi-
fication of Languages. This section provides an informal summary of RFC 3066.

This syntax, which is summarized below, is also used to identify languages in
XML, according to the W3C document Extensible Markup Language (XML) 1.1;
see the Bibliography for more information about these documents. An empty
string indicates that the language is unknown.

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