Is it supposed to just be window-dressing? Is it supposed to promote the (outdated and highly dubious) notion that all educated people speak French? Is it just for the author to show off? Whatever the reason, it's a highly obnoxious practice and it doesn't improve the article.
For mathematicians of a certain age knowing how to read French, English, and German is pretty common since often mathematics papers were only available in one language. So, my guess is that because of when the author was educated this is just normal for him.
Would you have preferred Latin or German ? Leibniz used them too.
I could get by with German, but that's beside the point: the essay is written for an English-speaking audience, so it's stupid to insert extended passages of foreign text without providing translation.
You shouldn't jump to assuming malice when someone's aesthetic tastes differ from your own.
By the way, the quote means (my own translation):
> God chose the one which was the most perfect; that is, the one which whose hypotheses were the simplest, and whose phenomena were the richest.
> But when a rule is very intricate, that which follows it seems irregular.