TPdfProperties.Language Property
Language identifier specifying the natural language for the document. This should be a standard specifier like "en-US".
Note that the language will be used by text-to-speech engines to read text out loud, so it is recommended to set this property.
You can see a list of languages here: http://msdn.microsoft.com/en-us/library/ee825488(v=cs.20).aspx
Syntax
Namespace: FlexCel.Pdf
public String Language { get; set; }