Having spent 22 years in Las Cruces, New Mexico, Hatch chile is always my preference. However, you can substitute poblano, anaheim, serrano, jalapeno. You can also substitute frozen diced or whole roasted chile. Use about 1 cup, and add to the processor bowl with the tomatillos. Make sure they're roasted, seeded, and peeled first!
If you're not interested in making your salsa verde, choose a good quality commercially available one (recommended options in post).
I like small pieces in my tacos. I diced the lengua in 1/4" to 1/2" dice. It's easier to eat and to keep the tacos from falling apart in a fresh tortilla.
I like to heat tortillas on a hot skillet (comal). This can be tedious if you're cooking for more than 2 people. You can put them in the microwave in a tortilla warmer or wrap them in foil and warm them in the oven. For more on keeping tortillas warm see this link.
The leftover salsa verde will keep for several days in the refrigerator, but I'll bet it won't last that long! This recipe assumes you have already cooked tongue. For more on preparing tongue using a pressure-cooker (or other method), see Provencal Beef Tongue Sandwiches.
Macronutrients (approximation from MyFitnessPal.com): 520 calories; 24 g protein; 40 g carbohydrates; 30 g fat. I hesitated to even include as there are too many variables. I used 3 corn tortillas, 4 ounces of lengua, 1/4th of an avocado, 1/2 ounce cotija, 3 tablespoons salsa verde and caramelized onion. This at least gets you in the "ballpark," and Google wants the calories ;-)