PdfDocumentInfoLanguage Property

Gets or sets the language of the PDF document as a BCP 47 language tag, for example "en-US". The value is written to the document catalog and, when a PDF/UA or PDF/A standard is active, to the XMP metadata. A value set here replaces the language declared by the HTML document; left empty, the language declared by the HTML document is used. The default is empty

public string Language { get; set; }