I doubt it would be called English, so what is it? And champions from different regions have interacted before so we know that each region speaks their own language (i.e. Ur-Nox for Noxians) but also another language. Although I could be wrong about all this.
I would assume, for lack of a better word, "Common". There are, however, several other languages that some characters speak in game or it has been mentioned that they speak it out of game:
All of them speak Japanese, it's the most badass way to represent them when fighting and using their attacks
I'm guessing Valorinian is what we define as 'English', but there are plenty of other languages (Shuriman, Ur-Noxian, Buhrutian etc.)
