Represents the "Lang" entry of the optional content usage language dictionary object.

It is a required entry defined as part of the PDF 1.0 specification.

It contains a string representing a PDF string object.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 102, page 232.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 100, page 284.