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I asked this question already here, someone suggested to move the discussion to this website. I hope some of the Italian users (at least) can help me out with this curiosity.
I have always been used to the Italian pronunciation "de ics" of the differential dx. In principle there would be nothing wrong in saying "di ics", as the letter "d" is pronunced "di", but then why do Italian mathematicians say "de ics"?
Three main differential calculus notations exists:
Others exist but they are less used (https://en.m.wikipedia.org/wiki/Notation_for_differentiation)
The notation you mention is Leibniz's notation, which favored publishing philosophy and maths works in Latin and French (https://history.stackexchange.com/a/45828/4245). The notation itself has been published and explained by Leibniz mainly in Latin, which would not read dx as "de ics".
Far from being a reliable answer, I guess dx could be pronounced as in French "dè ics" because that's the original language of the earliest widespread text book around the subject: de l'Hôpital's Analyse des Infiniment Petits pour l'Intelligence des Lignes Courbes
I think that for whoever speaks French (even as a foreign language as in my case) it comes natural to read it that way.
The book was fairly successful and had great resonance throughout the mathematical community.
