“Pape Satàn, pape Satàn aleppe!”
so Plutus, with his grating voice, began.

[“Pape Satàn, pape Satàn aleppe!”,
cominciò Pluto con la voce chioccia.]

Dante Alighieri (1265-1321) Italian poet

The Divine Comedy [Divina Commedia], Book 1 “Inferno,” Canto 7, l. 1ff (7.1-2) (1309) [tr. Mandelbaum (1980)]
    (Source)

There is a conflation in this speaker between Pluto, Roman God of the Underworld (modeled after the Greek Hades), and Plutus, Roman God of Wealth (both given as Pluto in Italian). The Romans themselves sometimes conflated the two figures (wealth, in the form of precious metals and gems, coming from below the ground). Given the sinners in this Circle (hoarders and wasters), the connection with wealth is probably intentional.

The actual words spoken remain something of a mystery. Dayman notes the phrase has "employed the ingenuity of commentators," and Butler that it has generated "commentary enough to fill a very large volume," but Sayers notes of the explanations "none of them is very convincing." Musa says, "this line, while it has never been interpreted satisfactorily, has certainly been interpreted variously." The line even gets its own Wikipedia entry.Earlier translators try to make sense of it; later ones just record Dante's original words and then speculate in footnotes.

The connection between pape and papa (Pope) is considered significant by most scholars, though papae is also Latin for an exclamation of surprise (παπαί in Greek), like "Oh!" Satan is the Hebrew term for "Adversary" and usually used to represent the master of Hell (though the name is not used lower down in Inferno when he is actually encountered); some scholars suggest Dante the Pilgrim, himself, is being called an adversary/enemy by the speaker, who acts as a guard. Some have tried to connect aleppe to the first letter of the Hebrew alphabet, aleph, with an implication of primacy (as of God, or, presumptively, Satan) or an an exclamation of grief or pain (as it was used in Medieval times).

In sum, this seems to be either infernal gibberish, or (as Virgil appears to understand it) some metaphysical jargon invoking the Devil in surprise or anger over a living mortal's intrusion. I'm mostly just amused by the array of accents / diacritical marks various translators use.

(Source (Italian)). Alternate translations:

O Satan, Satan, Oh alas! exclaim'd
Pluto, expressing both surprise and dread.
[tr. Rogers (1782)]

"Prince of the Fiends," a voice exclaim'd, "arise;
Behold thy realms expos'd to mortal eyes!"
[tr. Boyd (1802)]

“Ah me! O Satan! Satan!” loud exclaim’d
Plutus, in accent hoarse of wild alarm.
[tr. Cary (1814)]

"Ho! Satan, ho! -- Ho! Satan, ho! -- alas!"
Plutus began with stammering accents hoarse.
[tr. Dayman (1843)]

"Pape Satan! pape Satan, aleppe!" began Plutus, with clucking voice.
[tr. Carlyle (1849)]

"Pape Satan! Pape Satan! Aleppe!"
Began then Pluto, with affrighted voice.
[tr. Bannerman (1850)]

"Papè Satan, papè Satan, aleppe,"
Plutus began with raucous voice to cry.
[tr. Johnston (1867)]

"Papë Satàn, Papë Satàn, Aleppë!"
Thus Plutus with his clucking voice began.
[tr. Longfellow (1867)]

"Pape Satan pape Satan aleppe," began Pluto with his clucking voice.
[tr. Butler (1885)]

"Ah, marvel, Satan! marvel, King of Hell!"
Pluto began with his hoarse strident shout.
[tr. Minchin (1885)]

“Pape Satan, pape Satan aleppe,” -- began Pluto with his clucking voice.
[tr. Norton (1892)]

"Pape Satan, pape Satan aleppe," Pluto began with grating voice.
[tr. Sullivan (1893)]

"Papè Satàn, papè Satàn, aleppè,"
Plutus with voice discordant made beginning.
[tr. Griffith (1908)]

“Pape Satan, Pape Satan, aleppe,”
began Plutus with clucking voice.
[tr. Sinclair (1939)]

"Pape Satan, aleppe, pape Satan!"
[...] Plutus thus with clucking noise began.
[tr. Binyon (1943)]

"Papè Satan, papè Satan aleppe,"
Pluto 'gan gabble with his clucking tongue.
[tr. Sayers (1949)]

"Papa Satán, Papa Satán, aleppy,"
Plutus clucked and stuttered in his rage.
[tr. Ciardi (1954)]

"Pape Satàn, pape Satàn aleppe!" Plutus began with a clucking voice.
[tr. Singleton (1970)]

"Pape Satàn, pape Satàn aleppe!"
the voice of Plutus clucked these words at us.
[tr. Musa (1971)]

"Papè Satan, papè Satan aleppé,"
Plutus began, in his raucous voice.
[tr. Sisson (1981)]

"Papè Satan, papè Satan, aleppe,"
Plutus began in a gutteral, clucking voice.
[tr. Pinsky (1994)]

"Pape Satàn, pape Satàn aleppe!" began Plutus with his clucking voice.
[tr. Durling (1996)]

"Pape Satan, pape Satan aleppe," Plutus, began to croak.
[tr. Kline (2002)]

"Pappy Satin Papish Satan Alibi!"
barked Pluto in his fluent poppycock.
[tr. Carson (2002)]

"Popoi Satan, popoi Satan! Alezorul!"
So Plutus -- shrill voice clucking on -- began.
[tr. Kirkpatrick (2006)]

"Pape Satàn, Pape Satàn, aleppe!"
burst out Plutus in his raucous voice.
[tr. Hollander/Hollander (2007)]

"Satan's the Pope, Satan's the Pope, hurray!"
Plutus began, clucking like a mother hen.
[tr. Raffel (2010)]

"Pope Satan, Pope Satan, Alley Oop!"
Plutus spit this out in his raspy voice.
[tr. Bang (2012)]

"The Pope pops Satan, Satan pips the Pope,"
Plutus barked raucous nonsense.
[tr. James (2013)]