Esperanto is a constructed auxiliary language. It's creator was a Polish doctor name of L. Zamenhof. He created the language to make international communication easier. His goal was to construct Esperanto in such a way that people can learn it much more easily than any other language.
Back in the Middle Ages, in Europe, those who learned to read and write, learned to read and write (and speak) Latin, whatever their native language might be. This meant that people could talk and exchange ideas using Latin.
The rise of nationalism, or was it earlier, people started to read and write in their various native languages and Latin became sidelined, so losing the former ease of international discourse. These days, another language has grown up to replace Latin, and despite the best efforts, it's not Esperanto, it's English. Nowadays people across the world all learn English as a second language. So English has become the new Latin.
Many university courses in Europe require an advanced level of English, even though English is not the country's native language, and of course, English is the modern day Latin which is used in business communication.
So to the question: "How come I speak English?" English speakers should count their blessings, it's not an easy language to master if you didn’t grow up using it, and yet it has become an important key to success in all kinds of endeavours all over the world.