I'd just like to underline the fact that the epigraph is not in English, right. It's written partly in Latin and partly in Greek (the Sybil's answer).
Whereas most learned readers will be able to read the Latin sentence, I don't think that everybody will understand the Greek ones straight away. And that is also the most important part of the epigraph.
So, how is the reader to interpret it?
 
I don't think Eliot believed that everybody would understand the whole epigraph right on the spot. And I don't think he wanted us to think of the whole Satiricon story when reading it.
Think of Tolstoj's epigraph at the beginning of "Anna Karenina" -- if I remember correctly, it was taken from the Bible, and it's about divine nemesis. You don't have to understand it before you read the novel. You will afterwards, though. Epigraphs are just sententiae, summarizing the primary feeling which the work is based on.  
 
Usually, epigraphs in Western literature come from well-acknowledged literary 'authorities', such as the classics or the Bible. So, I think that is mainly the case here, as Carrol Cox has pointed out already, that it was put there mainly to convey that sense of 'this has all happened before'. An epigraph in English, taken from Conrad, would not have been that weighty and authoritative. The languages themselves are weighty -- Latin and Greek.  
 
-- Sara