First, it's not true to assume that all foreign bits in Foucault's Pendulum were left untranslated in the English version:
However, Latin is more familiar to Italian, French, Spanish,
Portuguese, or Rumanian readers than to Britons or Americans. For
this reason, Weaver, with my approval, sometimes shortened some
long quotations and nonchalantly inserted some English paraphrases.
It was a process of domestication that attempted to preserve some
archaic aspects of the text.
Umberto Eco, Experiences in Translation
(all unsourced quotes that follow are from the same piece)
Let's also state that if Umberto Eco is well acquainted with his translations, he should not be considered as the author or even as "approving" his translations. When speaking about a remarkable German translation, he states:
my contribution consisted not in providing an ‘authorized’ interpretation but
in encouraging an alternative solution - as if I were a co-translator.
Umberto Eco is much against statements of intent and helps his translators by emphasizing textual evidence rather than by imposing his own interpretation of his work.
Reading another tribute by Eco out-rules another option: it was not either to keep the translation "true to word":
He [William Weaver] was a great translator because he also knew that to translate the meaning, one must dare to reject the literal translation in order to conserve the effect or the deeper sense of the text. For reasons of space, I am limited to relating one amusing memory, of a time in which we tore the text apart in order to render a simple play on words, a wordplay that was already difficult for Italian readers.
Umberto Eco, Ciao, Bill
The same also gives us a credible explanation for the Italian words present in the text:
he sought to accurately render the fluidity, the rhythm, the lexical richness, and the sound of the text
Umberto Eco, Ciao, Bill
But an explanation is not enough: we want to know when, not why Weaver chose to keep Italian words... I'm currently re-reading the great interviews Weaver gave to the Paris review in hope of finding something, but this is the closest I got:
I start at page one, and I may even start at page one this time because I’m thinking about this problem all the time. Briefly, my modus operandi is simply to begin and work as fast as possible, leaving gaps. If there is a problem I will make a big parenthesis, indicating that I have left a sentence out. Or I will translate it and write, He was happy / glad / delighted / overjoyed, and decide later. If there’s something I don’t really understand I will leave it. I will type it in Italian and put it in boldface, so that when I see it I’ll know that it’s a real problem, which I’ll have to discuss with Eco. After having done this very rapid (i.e., in a month or two) first draft, I print it out; then I arm myself with a box of Ebony soft pencils and good erasers, and I go over it painstakingly with a pencil. I fill in the blanks, make the choices and change things. I have a big legal pad beside me and at the top is written, Ask Um; then I write: pg. 35; pg. 10; and the things I want to discuss with him.
William Weaver, The Art of Translation n°3, Paris Review
This makes for a very poor explanation on the finished work, however - given the extremely high quality of his translations, it's unlikely that they were left incomplete, so I don't consider this a finished answer. Rather, a commentary much too long to be published as such!