How can I translate “talent” into Ancient Greek?

Well…

Talent as is  in the ancient coin is τάλαντον, as Haggen Kennedy said. Talent as in being talented, not so much. The googles tell me that the modern sense is Mediaeval Latin, with an allusion to a parable in the Bible: Online Etymology Dictionary . As far as I know, that metaphorical extension did not happen in Mediaeval Greek—it’s certainly not given in Lampe’s Patristic lexicon; and the Modern Greek ταλέντο is a borrowing back from Italian.

So what of the modern sense of talent? English-Greek Dictionary is an online version of Woodhouse’s English-Greek dictionary: it gives:

• δύναμις: capacity
• δεινότης: cleverness
• φρόνησις: mental powers
• εὐφυής εἶναι εἰς..: aptitude for…

(But that is a secondary sense of εὐφυής, and in LSJ the corresponding noun εὐφυΐα still means “shapeliness; good disposition; fertility”.)

LSJ uses “talent” as a translation for:

• δῶρα: (natural) gift
• μεγαλοφυΐα: genius

Try some synonyms like “aptitude” on Woodhouse…