You don't expect to find decent Mexican fare, much less great Mexican, at a busy Littleton strip mall. But at El Lucero, a small but buzzing joint where the specials of the day are illegible on the dry-erase board and few of the employees speak English, the food needs no translation. In any language, your best bet is the tacos al carbón, three soft tacos filled with beef so soft and salty it's like eating buttery popcorn. Fresh pico de gallo comes with the tacos, and its sharp jalape?o bite is just right to play off the meat. For some added oomph, order a side of the green chile, and ladle on spoonfuls of this bright-orange fiery mix packed with big pieces of pork.