Complex sentence: In recent years Eastern Catholic Churches have been returning to ancient Eastern practices in accord with the Vatican II decree, Orientalium Ecclesiarum, which mandates that authentic Eastern Catholic practices are not to be set aside in favour of imported Latin Rite ones.
Split into simpler sentences: In recent years Eastern Catholic Churches have been returning to ancient Eastern practices in accord with the Vatican II decree, "Orientalium Ecclesiarum". It mandated that authentic Eastern Catholic practices were not to be set aside in favour of imported Latin Rite ones.