For my current campaign I'm exploring lore options regarding planar travel for NPCs and PCs. Where in the books can I find more information about which languages are common in which planes?*
I'm aware that the Monster Manual states which languages a creature knows and/or understands. But the details don't offer me demographics on how common certain languages are. Or am I missing something in the book?
*My campaign takes place in a personal adaptation on the planes of the Forgotten Realms.