At 13000 Atlantis Time, there was a change.

This was the first time the priest class ever moved from their home temple sites. This may sound mindboggling for modern people, but the immortal elites never travelled anywhere. The reason for this is simple: you don't spend 2000 years building a temple site what fits your life like a glove fits your hand, just to travel somewhere else. They built up their own area and in between were just wilderness, farmlands and highways. Seeing something new wasn't an issue because everyone's 3rd eye was open, so they could just Remote View any location.

At 13000 AT, teleportation was invented. This allowed for instant travel via magic circles, to any major temple site in the world. This lead to a creation of a world-wide culture, where the priest class united their efforts to adapt the central temples located at intersections between areas, so that anyone would be able to live there and it would suit their needs just the same as their home area.

Because of the stiffness of each area egregore, uniting into a single culture was a huge effort. The first united site was built so that all local leaders could meet and interact, without their local egregores causing the world to literally crack, which was a fear at the time. After constructing this site from afar, and it synced everyone into one location, the layout was copied and constructed all over the world.

This is how Atlantis developed a single culture and a single language, for all of the world.