The most standard language that unites the world in trade and diplomacy, and divides it just as easily. Speakers of this language come from all races and live almost everywhere in the world, this will be the standard language that creatures speak in heavily populated areas. The script for Common is incredibly simple and because of that, it is considered the simplest language in the world to learn for reading and writing.
The earliest appearance of this language is hard to dictate, the Human Empire
claims that this is their native tongue while others argue that there is a more complex language that gradually devolved into the Common speech. Regardless, Common has been around for the extent of history that the Elder Races
have recorded. It is widely accepted to have spread due to the adventurous and wide-ranging habits that the Human
race exhibited. While their native homeland is the Great Plains
have spread to have some sort of presence in all parts of Isekai and with them, the Common language has spread.
There is no particular advantage or disadvantage to speaking or knowing this language among the world's many cultures. The greatest reason to learn this language is that so many other creatures in the world use it to communicate with each other but for the purpose of deciphering ancient runes, learning magic, or communicating with ancient beings this language will hold no particular value.