Were Adam and Eve able to talk to each other?
Eve talked with the serpent.
They could talk.
What language did they use?
I don't know. We do know there was one language on earth until the Tower of Babel, right?
Possibly, Hebrew made it through it all. Adam and Eve eventually gave rise to Jesus. So, may be Hebrew was a common link throughout the genealogy to Jesus.
What was their range of vocabulary?
I would say > at first, Adam and Eve were perfectly created beings, without sin. So, their capabilities were not being spoiled by sin. So, their range or potential was what we may not have experienced.
From them have come humans able to go to the moon and do brain separations of Siamese twins. So, even after sin has come into humans, still humans have the various abilities that we do now see. So, before their fall . . . they were even more, in who knows what abilities and who knows what vocabulary potential. Even if they had a great range, I would say their potential was for much more. But sin degraded them, including to being able to have shame about what is not a rightful cause for shame > they got very mixed and messed up.
I'd like to mention > since I have trusted in Jesus, I would say I have become much more creative, and this mainly working in loving any and all people. I just used people; now I can get close to women and not only to try to use them. I have become more and more easily free of how worry and anger used to degrade me and keep me from loving. I can make much more loving and creative use of abilities I have had, and of my education; because love creates.
Love is a language, by the way, isn't it?
Even though they were perfect as creatures, they were not perfect in God's love which makes us able to say no to wrong stuff.
Their range of love was very limited, then. Range of vocabulary and linguistic ability is very limited, compared with ability to relate and communicate with God and people.
Did they have words for things they didn't know about?
I think we could ask if they knew the word "love" and all this can mean.