Foucault's Pendulum by Umberto Eco was originally written in Italian. Most of the book was translated to English, except for the parts that weren't in Italian to begin with. 

I understand that decision; however, at other places in the book, specific phrases, sentences, and words are left in their original Italian. Is there diegetic significance to those phrases? If not, by what method was the book translated that led to certain untranslated words & phrases?