Sprint iPhone 5s in Russia?

I have a Sprint iPhone 5s that has supposedly been unlocked by Sprint for International use. Does this mean that a SIM card from another country will work in the phone while abroad? My sister lives in Russia and I would like to send her the phone. Can she just pop in a SIM card since Sprint has given me confirmation that the device has been "unlocked for International use." Is there a way to check IMEI for Russian networks? Please assist! Thank you! 🙂