Because her reasoning was "English is the only language I know." On one level, this is a practical answer - but it also misses the implied point that if the whole world is speaking one language then of course you, being part of the world, would know it too.
This of course begs the question of what a better answer might be. You could say something like "Spanish, because it is the language of romance", or "French, because it sounds so beautiful and elegant", or even "Chinese, because it's spoken by the most people in the world already".
Or, for extra points, you could transcend the question with, "If the whole world spoke one language, it wouldn't matter which one it was, because it would create an enormous opportunity to bring about peace and understanding between people everywhere."
But no, she said "I only know English, so...", which I have to admit is at the very least a bit of a let down for an answer.