χοιρινός, χοιρινή, χοιρινό is an adjective which means made of pig. The full sentence should be Το δείπνο είναι κρέας χοιρινό (The dinner is meat made of pork), but in Greek very often the noun can be omitted and the adjective indicates the meaning of the omitted noun, therefore Το δείπνο είναι χοιρινό is correct, accepted and preferred by the Greek speakers. In the second sentence χοιρινό is considered a noun, not an adjective.
Is there a difference between "το δείπνο είναι κρέας χοιρινό" and just "δείπνο είναι κρέας χοιρινό"? If not, then "dinner/supper is pork" is the correct translation in English- the article is not used in this case (even more natural-sounding would be "pork is for dinner/supper" or "there is pork for dinner/supper")