Common (As the name suggests) is one of the most, if not the most universal languages in Azeroth. Many non-human races are known to speak Common, especially within the Grand Alliance as the dialect acts as one of the primarily languages within it.
Once called Arathi by the people who used it, Common derived its new name during the era of the Empire of Arathor, when the House of Lothar ordered a singular language to be used among the empire's trade. This language, Common, was then spread to every sub-sect of the Kingdom and taught en masse. It quickly replaced other dialects in the empire, namely Galui, Tirassi and Alteic. While these languages would go on to survive, some in limited capacity, Common ultimately fulfilled the Emperor's goal by creating a unified language in the empire.
So wide spread was Common that it outlived the empire it's self, going on to continue as the language used by most of humanity even after the Seven Human Kingdoms were formed. In time, Common took the place as the neutral language between communication with the High Elves of the Kingdom of Quel'Thalas, communication with the Dwarves of Khaz Modan and the Gnomes of Gnomeregan. As such, Common lived to its name, as it became the language of commerce and one spoke among the entirety of the Eastern Kingdoms.
In modern day, Common continues to be wide spread, having become the most spoken language in the entire world. Spoken by each race of the Alliance, some of the Horde races, and used extensively by the Goblin cartels, common has transcended its roots as a simple dialect of hill wanderers.
Common has no real world equivalent in spoken form. There have been times where it is compared to Old English, though actual examples of it being spoken have no translation or similarity to known languages.
In written form, it was previously random scrawls or illegible due to pixel size. As of Legion, however, Common has been represented in the old germanic language Elder Futhark, the oldest form of runic alphabet. This can be seen on the Lion's Rest monuments, which can all be translated using this language. When translating the monuments, the central monument on Varian's tomb reads an epitaph, while the rest are actually easter eggs of Blizzard employee names.