The succubus, shivarra, felguard, and wrathguard can tell you the language. The voidwalker and voidlord can tell you the purpose. The observer can tell you both.
Most minions have unique responses, which I've copied below, beginning with the introduction. This information is from the PTR, so things may change between now and live.
- Imp/Fel Imp: ...yeah? Sup, boss?
- Voidwalker: Dismiss me... I wish to return to the void.
- Felhunter: It looks at you blankly.
- Succubus: Hello Master... what kind of service can I provide you today... ?
- Fel/guard/Wrathguard: ... what do you want?
- Voidlord: You wish to speak with me...? Unusual...
- Observer: Ah, Master <Name>, how might I be of assistance?
- Shivarra: What do YOU want, you miserable excuse for a <race>?
<What can you tell me about this tome?>
- Imp/Fel Imp: Hmm... I have no idea.
- Voidwalker: Words are not the way of the void. I do not know... it appears to connect two... stars together...
- Felhunter: The felhunter just tilts its head and stares at you strangely.
- Succubus: What? You just want me to look at some dirty old tome? Ugh, how disappointing.
This is the script of the Nathrezim, what you call dreadlords. They rarely use it outside of their home.
- Felguard/Wrathguard: Hmm... I recall seeing such a script on the Nathrezim homeworld. I cannot read it and I doubt any of your kind could either.
- Voidlord: I cannot read the words... but the magic points to a world that exists no longer. It was destroyed by the Legion.
- Observer: Of course! This codex is written in the ancient Nathrezim tongue. Nathrezim, or the Dreadlords, as you call them, were once an enlightened and powerful race. Their skill in summoning and gateway magic was unsurpassed by any in the Great Dark Beyond.
This particular tome contains instruction to open a gateway to another world. Sadly, I fear that world no longer exists. It was destroyed as a show of power during the enslavement of Xoroth.
The tome is useless, I'm afraid. No <race> warlock could open such a gateway alone.
<I had no idea you were so well informed.>
It is the way of the Observer to bear witness to all things that come to pass. So many delicious magics have I tasted at your side, master.
- Shivarra: No. I won't help your kind read an ancient Nathrezim text. Now, release me!
<Fascinating...>