Literally translated (more or less translating word for word), “le plus affable des hommes” could indeed be rendered “the most affable of men.” A beginning translator, or a student of French, would tend to translate it this way. But an experienced translator (i.e., an expert translator [I am one myself, nationally certified in both directions and with over 20
years of experience]) knows that this kind of “straightforward” translation is not usually the best translation. The idea is not to make the translation match the original as closely as possible (in word choice, word order, grammatical structure, etc.): the point of translation is to take and idea as it is naturally expressed in its original source language and, essentially ignoring how it was expressed originally, recreate that same idea again in a way that sounds natural, would be readily understood in the same way, and will have the same impact in readers of the target language.
In this case “le plus affable des hommes” (literally “the most affable of THE men”, [des = *“de les”, which would be incorrect]) could more appropriately be rendered into English in various, better, ways, with the choice depending on any number of factors about the text, its source, its intended effect on the reader, and more. In English, most native speakers have never heard of the English word “affable” and couldn’t tell you what it means. We would also never say “the most [insert adjective] of THE men.”
So, taking in the idea that the original is conveying to French readers, then letting go of the words to recreate that idea in a way similarly well understood by an average native English speaker, there are many better translation options:
”the kindest person ever”
”the most easy-going person you would ever meet”
”the nicest person I ever met”
”the friendliest person I ever knew”
”the friendliest of men”
”the nicest guy ever”
”as kind-hearted as they come”
and many other possibilities, with the best choice(s) depending on the literal context (words surrounding the original phrase) and on other contexts (where, by whom, and for whom it was originally written and now intended; the originally intended effect on the reader; the real-world result that the original writing was intended to produce;…)
The more that can be known about the context, the better we can choose the translation that would be the best translation. Regardless, at least understand that hewing slavishly to the original word choices and order is typically how you land on a bad translation. Good translations translate the idea/meaning in a way that will sound natural in the new language and will have the same meaning and effect for the new readers that the first-language version had on the original readers.
