The Dragonkill War wiped out the Orlanthi people of Dragon Pass in 1120. For more than two centuries, the people on the north side of Dragon Pass (beyond the Deathline) had no contact with the people on the south side (beyond the Crossline). There was no trade, no travel, no communication.
The Orlanthi on the south side of the Crossline came under heavily cultural influence from Esrolia, to the point where their dialect was mutually intelligible with Esrolian.
Meanwhile, the Orlanthi on the north side of the Deathline were largely subsumed into Sairdite, a Thelayan language with many by Dara Happan influences. The dialect of Sairdite spoken in Dragon Pass is called Tarshite, but is mutually intelligible with Holayan, Aggarite, and Imtherite (probably half chance).
For more than two centuries these languages developed without any influence on each other. During the resettlement of Dragon Pass, these two groups no longer spoke mutually intelligible languages – a speaker of Heortling at 50% understood Tarshite at 10% (but understands Esrolian at 25% – enough to get by with).
Thus even though both groups worshiped Orlanth, Ernalda, and the Lightbringers, they were easily seen and identified as different peoples and nations.
Does this explain why Tarsh was willing to be cooperative with the Lunar Empire due to a shared Dara Happan origin in their languages? No. Much of Tarsh (Dunstop, Bagnot, Slavewall) refused to support Hon-eel and her dynasty and broke away for almost a century. They submitted to the Lunar dynasty because they were defeated in battle and their cities occupied by troops.
How does language work when speaking to the spirits of your ancestors and in Heroquests? Your ability to communicate with spirits during a worship ceremony is courtesy of the magic of that ceremony. No special language needed.
Your ability to communicate with the ghost that haunts that Second Age tomb is based on having a language in common – Spirit Speech or some language known to you both.