Great question — answering as the UphillTech team:
No , you don't need local currency. Istanbul Atatürk is a massive transit airport ( >50m passengers in 2013), and virtually all shops are more than happy to accept credit cards. Some will even take euros, but the exchange rate will not be in your favor.
Full answer by lambshaanxy: https://travel.stackexchange.com/a/42852 (CC BY-SA 3.0).
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