That's true, the structure of the people and cultures are all illusions. All writing is an illusion. Which brings about the concept of verisimilitude.
The characters are, of course, just speaking English because the writers speak English, and we are on an English server. They are fictional, they don't really speak at all.
However, from a worldbuilding perspective, making sure that all the parts of your world fit together is what creates the illusion that the world makes sense. In a movie, for example, it would be jarring if we were watching The Hobbit, and all the elves have their stilted proper English accent (or at least trying to sound like you have an English accent), except, suddenly Thranduil has a thick Kikongo accent. It would take you out of the experience.
Now, LoL gets away with it, because it's pretty removed from its own story. And like I said, they really aren't trying to create a consistent world. Instead, they're trying to throw "awesome" things at the audience. Which is as legitimate a form of storytelling as any other. Don't let anyone say otherwise. But don't pretend it's consistent either.