add the guestbook plugin with code GUESTBOOK, POSTFORM

add following to your template (example for 2 extra languages italian, german. You have also to change

the templateFile=fileadmin/template/guestbook_de.tmpl for each of the language in a condition if needed.

NOTE: THE SECOND DEFAULT HTML TEMPLATE SEEMS TO BE BROKEN. You can fix that replaceing the ###POST_EMAIL### with ###POST_EMAIL###):

plugin.tt_guest {
  templateFile=fileadmin/template/guestbook_de.tmpl
   
  postform {
    labelWrap.wrap=|
    REQ.labelWrap.wrap=|
    dataArray{
      10.label.lang.de=Titel
      20.label.lang.de=Kommentar
      30.label.lang.de=Name
      40.label.lang.de=Email
      50.label.lang.de=Homepage
      60.value.lang.de=Eintragen
       
      10.label.lang.it=Titolo
      20.label.lang.it=Commento
      30.label.lang.it=Nome
      40.label.lang.it=Email
      50.label.lang.it=Homepage
      60.value.lang.it=Invia
    }
  }
}

#Fehlermeldung auf zb Deutsch setzen
plugin.tt_guest.postform.badMess=Folgende Felder müssen ausgefüllt werden: