Common is a language invented by Esperanto during its founding with magical properties. It was supposedly created by its founders, some of whom were powerful spellcasters.
If a speaker of a verbal language understands a language and comes into contact with Common, even a single sentence, they will learn an equivalent amount of common to their understanding of the other language within a month.
A baby who hears common will start to babble common words as frequently as their parents words. An elderly individual will suddenly learn the language with the same degree of sophistication as their native tongue.