Everyone speaking common was caused by a blunder by one of the most powerful wizards of all time.

Long ago a powerful wizard and then lich who's name and original goal have been lost to the sands of time traveled the world seeking knowledge and ever greater power.  They were going to bend reality to their will and create the most powerful version of the wish spell ever created.  This spell, a ritual based on a singular focus of a centuries of work and toil and unimaginable sacrifice, would not be a normal wish.  No, this version of wish, his all-powerful wish would allow him to finally, and permanently, shape the world to their liking.

They had been frustrated at times, having to learn a new language which took time or constantly use magic to comprehend everyone's language which could have been channeled to his all-powerful spell.  But that was the price they were willing to pay, one of the lesser sacrifices for the greater goal.

At the penultimate moment, the wish spell ritual that had been constructed over a millennia was ready.  Having carefully conserved all of their energy for this moment, not bothering to cast a spell that let them speak other languages they started speaking the words to enact the spell.

A servant interrupted them with a message in a language the wizard had not bothered to learn.  Frustrated, he spoke the words that would change history "I wish everyone spoke a common tongue".