Place half of the cucumbers, apples, and tomatillos into the bowl of a food processor. Add all of the grapes, walnuts, yogurt, grape juice, vinegar, and mint. Pulse 9 to 10 times. Combine with the remaining half of the cucumber, apple, and tomatillo. Season with salt, to taste. Chill for 2 hours in the refrigerator before serving. Serve as an appetizer or soup course.