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It's hard to answer- there has been English outside the British Isles for hundreds of years, but even in the nineteenth century, the language of diplomacy in Europe was still French. The rise of English was in the the last two hundred years.
English became "the international language" for one very simple historical reason: the last two great global economic superpowers back-to-back since about the Battle of Waterloo have both been English-speaking.
The spread of English followed the expansion of the British Empire in the 19th Century. In the 20th Century, the rise of the US as a world economic and military power served to consolidate the pre-eminent position of the language.
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