What an interesting translation! Zed becomes zero, and then called the thirtieth letter. I wonder if "trentième lettre" is a standard phrase in French, or if this is whimsical.

It's interesting to compare this to something I stumbled on when I googled "whoreson zed": "Why do the British pronounced the letter Z 'zed'?" Amongst other things, this article claims that "zed" is from the Old French "zede" (ultimately from the Greek "zeta"). So why "zéro" instead of "zede"? Perhaps "zede" has an archaic connotation, I dunno.