Quintilian, Institutio Oratoria XI.2.50-51:
Ceterum quantum natura studioque valeat memoria vel
Themistocles testis, quem unum intra annum optime locutum esse Persice
constat, vel Mithridates, cui duas et viginti linguas, quot nationibus
imperabat, traditur notas fuisse, vel Crassus ille dives, qui cum Asiae
praeesset quinque Graeci sermonis differentias sic tenuit ut qua quisque
apud eum lingua postulasset eadem ius sibi redditum ferret, vel Cyrus,
quem omnium militum tenuisse creditum est nomina: quin semel auditos quamlibet multos versus protinus dicitur
reddidisse Theodectes. Dicebantur etiam nunc esse qui facerent, sed mihi
numquam ut ipse interessem contigit: habenda tamen fides est vel in
hoc, ut qui crediderit et speret.
For the rest there are many historical examples of the power to which
memory may be developed by natural aptitude and application.
Themistocles is said to have spoken excellently in Persian after a
year's study; Mithridates is recorded to have known twenty-two
languages, that being the number of the different nations included in
his empire [cf. Pliny the Elder, Natural History VII 24]; Crassus, surnamed the Rich [cos. 131 BC, was the commander in the war against Aristonicus of Pergamum, was defeated and was killed], when commanding in Asia had such
a complete mastery of five different Greek dialects, that he would give
judgement in the dialect employed by the plaintiff in putting forward
his suit; Cyrus is believed to have known the name of every soldier in his army,  while Theodectes
is actually said to have been able to repeat any number of verses after
only a single hearing. I remember that it used to be alleged that there
were persons still living who could do the same, though I never had the
good fortune to be present at such a performance. Still, we shall do
well to have faith in such miracles, if only that he who believes may
also hope to achieve the like. (Loeb: H.E. Butler, 1922)
For Mithridates VI of Pontus as a polyglot, see: Aulus Gellius 17.17, Pliny the Elder 7.24.88-90 and 25.3.6-7, Valerius Maximus 8.7, and, for a claim to knowledge of fifty languages, Aurelius Victor '(AD 360)'. These references are from Adrienne Mayor, The Poison King:The Life and Legend of Mithradates, Rome's Deadliest Enemy (Princeton University Press: 2009) 254 and 408n. 36.
The reference to Aurelius Victor is probably de viribus illustribus 76.1 ('incerti auctoris'): Mithridates rex Ponti oriundus a septem Persis, magna ui animi et corporis, ut sexiuges equos regeret quinquaginta
gentium ore loqueretur.
Xenophon Cyrus 5 is cited for Cyrus the Great's knowledge of the names of his officers and satraps. This must be Cyropedia 5.3.47.
OLD s.v. differentia -ae f. 2 b a different kind, quotes only this passage and explains '-as (i.e. dialects)'. Quintilian, Inst.IX.4.18 uses dialectos -i f. (OLD s.v. also cites Suet.Tib.56 and Maur.649 but no other instances): "As regards Herodotus, while his flow, in my opinion, is always gentle, his dialect has such a sweetness of its own that it even seems to contain a certain rhythmical power hidden within itself."
Terentianus Maurus (GL vi) line 649 reads: Aeolica dialectos autem mixta ferme est Italae. The context is a discussion of the digammon and ensuing Greek and Latin cognates.