Three main differential calculus notations exists:
- Newton's, favored in physics
- Leibniz's, favored in multidimensional maths
- Lagrange', favored in maths on a single dimension
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.