Spirits coming back to possess are coming from widely different epochs and regions. How come they all speak English?
The dead in The Beyond are mentally open to each other. So, unlike the mass resurrection in Riverworld, there was opportunity for the dead to pick up language from each other. [Below, Shaun implies it might be a vampirish acquisition.] The largest proportion of dead would be from the most recent, most populous generations. Therefore it would be logical to suppose that their language would become dominant, although I don't recall the point being made explicit.
In The Reality Dysfunction, the reporter Kelly Tirrel interviews Shaun Wallace, one of the returned dead.
“You went to hell?”
“Hell is a place, so the good priests taught me. This beyond was no place. It was dry and empty, and it was cruel beyond physical pain. It was where you can see the living wasting their lives, and where you drain the substance from each other.”
In another conversation, he talks of the other dead, asking if Hitler is still remembered:
“We remember Hitler. Though he was after your time, I think.”
“Indeed he was. But do you think he changed after he died, Mr. Halahan? Do you think he lost his conviction, or his righteousness? Do you think death causes you to look back on life and makes you realize what an ass you’ve been? Oh no, not that, Mr. Halahan. You’re too busy screaming, you’re too busy cursing, you’re too busy coveting your neighbour’s memory for the bitter dregs of taste and colour it gives you. Death does not bestow wisdom, Mr. Halahan. It does not make you humble before the Lord. More’s the pity.”