From Old High German swīn, from Proto-Germanic *swīną. In the plural, the expected Luxembourgish form is *Schwéng, which is indeed used in most compound words (e.g. Schwéngsfleesch “pork”). The unchanged plural would seem to be secondary, possibly by analogy with Päerd (“horse”) and Schof (“sheep”), which have regular unchanged plurals.
Schwäin m (plural Schwäin)