(note that in the following essay, I'm making a lot of things up - I'm not attempting to stick to what is known in canon)
The issue here is Planar Common, the dominant language in the City of Doors and other human-dominated planewalker communities on the planes. Where did it come from?
The Planewalker's Handbook (page 101) said that Planar Common descends from the Prime Common tongue, brought to the planes by prime explorers. In fact, it says "the earliest planar settlers from the Prime," which suggests that this couldn't possibly be the modern Common Tongue of Toril or Oerth or Krynn.