English is the national language of the vast majority of the population that plays on the NA server.
I don't include Greenland, as they are physically closer to the EUW server. The population of the entire Carribean is too small to cater to.
And then there is Mexico. Many of them do speak enough English to communicate in LoL, but refuse to. Others speak absolutely no English, and thus unable to communicate. Both are bad situations for the English-only speakers, no matter what country they are from.
Let's be honest, nobody is complaining too much about the French speaking Canadians, because they will speak in English when they need to. In fact, you've probably played with a French speaking Canadian and never knew it.
Meanwhile, you ALWAYS know when you've played with a Mexican.