Catechism of the Catholic Church §1205 — excerpt
"In the liturgy, above all that of the sacraments, there is an immutable part, a part that is divinely instituted and of which the Church is the guardian, and parts that can be changed, which the Church has the power and on occasion also the duty to adapt to the cultures of recently evangelized peoples."
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: John Paul II, Vicesimus quintus annus, 16 · SC 21