Tin, Greeks, Vikings, Oak Trees, Navy, Imperialism
An excellent topic, with many good answers....
But I think there is a progression of elements which made Britain great...
One, it is an island, and difficult to defeat and change, although it has been invaded and defeated and assimilated many times, just much less than areas with land borders with warring neighboring tribes.
It had tin, which attracted foreigners, probably Egyptians and Greeks over the millenia, who used it for bronze production, which was a major military advantage.
It had large oak trees, one primary ingredient for having a Navy. Even Egypt did not have large trees, or any trees, they had to conscript the Phoenicians who had Cedars of Lebanon, and then they had a large world trade Navy. Like the British became, with their oak trees and Navy.
With a Navy, it developed world trade.
With world trade, it developed an imperialism apparatus which spread its language use.
With world language use, it gave to the world a unified language of sorts, but please don't tell that to the French, a very proud and wonderful people, who with their Viking Normans invaded England and Ireland, etc. etc. But note how the defeated English STILL retained the language element over the invading Norman French?!
But the simple answer is ...most definitly "English language" to the world.