Isn't the intended audience of the article non-french speakers? So he writes it in a "universal" language so you could apply it on "how to learn <X> in 12 months" where X is your non-native tongue.
(For those who don't (or do) speak French, I tried to write this: "I'm the author. I thought about writing a French version, but there are 9000 words in the article. This would be an enormous task and probably not the best way to spend my time, since I write significantly slower in French than in English. Furthermore, I imagine that most people who are going to read this article are English speakers.")