"The girl eats spicy foods well."
Translation:그 여자아이는 매운 음식을 잘 먹어요.
Usually! In this case, the 그 is referring to a girl that people are already discussing, just like "the" works in English, so "that" or "the" are both correct.
In a real situation, you might just drop 그 여자아이는 altogether.