And as a side question: Is there any one thing we could go back in time and change, to make it be French or some other language instead?
The "biggest reasons" could potentially be up to debate, but a big reason is that, at it's height, the British Empire was the largest Empire ever. In 1922 it covered roughly 33% of the worlds land area, which was approximately 458 million people.
One of the primary aims of education in these nations was for them to learn the English language. Then even after the colonies were granted freedom, most of the leaders already spoke English, and so it was still taught. Now some 2 billion people speak English as a primary or secondary language.
As for the side question, I suppose if France had somehow beaten the British for control of the world, then most of the world would speak French.
hi! fyi, you'll find some info to get you started in the FAQ