ExportLanguageInformation

public bool ExportLanguageInformation { get; set; }

Document doc = new Document();
DocumentBuilder builder = new DocumentBuilder(doc);

// Use the builder to write text while formatting it in different locales.
builder.Font.LocaleId = new CultureInfo("en-US").LCID;
builder.Writeln("Hello world!");

builder.Font.LocaleId = new CultureInfo("en-GB").LCID;
builder.Writeln("Hello again!");

builder.Font.LocaleId = new CultureInfo("ru-RU").LCID;
builder.Write("Привет, мир!");

// When saving the document to HTML, we can pass a SaveOptions object
// to either preserve or discard each formatted text's locale.
// If we set the "ExportLanguageInformation" flag to "true",
// the output HTML document will contain the locales in "lang" attributes of <span> tags.
// If we set the "ExportLanguageInformation" flag to "false',
// the text in the output HTML document will not contain any locale information.
HtmlSaveOptions options = new HtmlSaveOptions
{
    ExportLanguageInformation = exportLanguageInformation,
    PrettyFormat = true
};

doc.Save(ArtifactsDir + "HtmlSaveOptions.ExportLanguageInformation.html", options);

string outDocContents = File.ReadAllText(ArtifactsDir + "HtmlSaveOptions.ExportLanguageInformation.html");

if (exportLanguageInformation)
{
    Assert.True(outDocContents.Contains("<span>Hello world!</span>"));
    Assert.True(outDocContents.Contains("<span lang=\"en-GB\">Hello again!</span>"));
    Assert.True(outDocContents.Contains("<span lang=\"ru-RU\">Привет, мир!</span>"));
}
else
{
    Assert.True(outDocContents.Contains("<span>Hello world!</span>"));
    Assert.True(outDocContents.Contains("<span>Hello again!</span>"));
    Assert.True(outDocContents.Contains("<span>Привет, мир!</span>"));
}