How do you say enjoy your lunch in spanish?

To specifically say the trarget sentence, you say, "Disfrute/a de su/tu almuerzo."

('e/'su' if you're being polite, 'a/'tu' if you're being familiar.) However, it needs to be said that non-dining Spanish-speaking people the world over have a standing custom of wishing diners, "Buen provecho," which connotes "good use [by your body of the food]." It's their way of saying, "Enjoy your ___________ (fill in the meal)."