Like every other SHN Maybe. QuestData can't be translated. QuestDialog contains all Quest Dialogs (Titles, Descriptions, Dialogs) so you only have to translate this.
And I just tell you ONE time now, cause this is ****** simple.
Open QuestDialog.shn, go to Tools -> Translate. Click Open to open the Same SHN, that has the language you want and then as Index use ID and as Translation Column Dialog.