Who knows? The book only gives us so much information. The rest is
left to your interpretation. I for one, was under the impression that
the ghost didn't even talk to the other watchmen. He just appeared in
front of them to get their attention and then it dissipated away. So I
don't think understanding is an issue, but rather it's who the ghost
decides to talk to.
And we know for a fact that the ghost isn't a figment of Hamlet's
imagination because Horatio and the others did, in fact, see the
ghost. How else would they be able to tell Hamlet about it? So we know
Hamlet isn't totally off his rocker.
My two cents. Do with it what you will.