Specifying the language with an /en/ in the URL isn't a major inconvenience, and allows for future expansion at the cost of 3 additional characters in an URL.

Including it says "I planned ahead and worked intentionally", which shows you care, which fundamentally is what this article is driving at.

And yes, documentation should generally be in English first - it's the current language of science and tech, just as it was when French, German, Latin, Arabic, and Greek proceeded it.