It's nothing to do with the amount of people speaking that language in the world and everything to do with economics - we have French and German localizations of the game because France and Germany are the two largest economies in Europe (even larger than the UK), and much much larger than Italy and Spain, so the cost of localizations for those later languages to SE just isn't considered justified by the projected amount of players (and thus returns on that investment).

And I doubt they would endorse a fan translation - they're usually very strict when it comes to that sort of thing, regardless of the benefit it may give.