The langauage known commonly as "trade" was developed for commerce between nations. As war became less frequent and trade more common, a language was needed to better communicate. The language spoken by traders from Berk was chosen, as it was the easiest to learn. It eventually became the most commonly-spoken language throughout the world, though many other languages thrive.
Commerce is also why most countries use the same calendar system. A universal system made the tracking of information much easier.