gtk:font-chooser-language (object)

object -- a gtk:font-chooser object
language -- a RFC-3066 format string representing the language

The accessor for the language slot gets or sets the language that is used for font features. See the pango:language-to-string function.

(gtk:font-chooser-language (make-instance 'gtk:font-button)) => "de-de"    

The gtk:font-chooser implementation is deprecated since 4.10. Use the gtk:font-dialog and gtk:font-dialog-button widgets instead.