Represents the "Lang" entry of the structure element dictionary object.

It is an optional entry defined as part of the PDF 1.4 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: 323, page 559.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 355, page 722.