Mortim

The language of the dead allows exactly what it sounds like, to speak with the dead. This is not the same as pulling a lost soul back into a body, but rather allows the linguist to speak to any spirits that are still present, as well as any Undead that can’t use their own vocal noises normally. But a corpse is a corpse, and can’t speak Mortim any better than any other language.

Mortim is included in the Gether Guide to Languages on DrivethruRPG:
https://www.drivethrurpg.com/product/328473/Gether-Guide-To-Languages