I like to believe that most people don't instantly resort to racism. Rather, it's a response that is provoked over time. For instance, most of the people on my friends list speak broken English at best, but that doesn't bother me in the slightest. They're doing their best to communicate, and hey, I can understand their intentions.
However, what absolutely irks me is when a player reveals that they grasp English to a degree, yet absolutely refuses to use. For instance, a player during Champion Select would be communicating just fine, asking about team compositions and such. Yet, the minute the game starts, they revert to their native tongue with their dual queue friend, and no matter how much you politely ask, they refuse to go back to English. They start missing MIAs, get ganked then call the other players noobs and such. In my experience, this is when the racism starts. Quite frankly, I get rude in these situations as I just want to elicit some sort of understandable response. It's especially infuriating when the players then end the game with, "reporting, I speak English". Well, it would have been nice if you communicated earlier.