On the use of Latin in the Roman Rite...

“The language proper to the Roman Church is Latin. Hence it is forbidden to sing anything whatever in the vernacular in solemn liturgical functions -- much more to sing in the vernacular the variable or common parts of the Mass and Office.”
-Pope Saint Pius X, Tra le Sollecitudini, November 22, 1903

"For the Church, precisely because it embraces all nations and is destined to endure until the end of time... of its very nature requires a language which is universal, immutable, and non-vernacular."
-Pope Pius XI, Officiorum Omnium, 1922

"The use of the Latin language prevailing in a great part of the Church affords at once an imposing sign of unity and an effective safeguard against the corruption of true doctrine."
-Pope Pius XII, Mediator Dei, 1947

"The Catholic Church has a dignity far surpassing that of every merely human society, for it was founded by Christ the Lord. It is altogether fitting, therefore, that the language it uses should be noble, majestic and non-vernacular."
-Blessed Pope John XXIII, Veterum Sapientia, 1962

"And We also, impelled by the weightiest of reasons the same as those which prompted Our Predecessors and provincial synods are fully determined to restore this language to its position of honor, and to do all We can to promote its study and use. The employment of Latin has recently been contested in many quarters, and many are asking what the mind of the Apostolic See is in this matter. We have therefore decided to issue the timely directives contained in this document, so as to ensure that the ancient and uninterrupted use of Latin be maintained and, where necessary, restored."
-Blessed Pope John XXIII, Veterum Sapientia, 1962

"The Latin language 'can be called truly catholic.' It has been consecrated through constant use by the Apostolic See, the mother and teacher of all Churches, and must be esteemed 'a treasure ... of incomparable worth.' It is a general passport to the proper understanding of the Christian writers of antiquity and the documents of the Church's teaching. It is also a most effective bond, binding the Church of today with that of the past and of the future in wonderful continuity."
-Blessed Pope John XXIII, Veterum Sapientia, 1962

#36 "The use of Latin, with due respect to particular law, is to be preserved in the Latin Rites."
-Second Vatican Council, Sacrosanctum Concilium, 1963

#54 "Nevertheless care must be taken to ensure that the faithful may also be able to say or sing together in Latin those parts of the Ordinary of the Mass which pertain to them."
-Second Vatican Council, Sacrosanctum Concilium, 1963

"The Latin language is assuredly worthy of being defended with great care instead of being scorned; for the Latin Church it is the most abundant source of Christian civilization and the richest treasury of piety... we must not hold in low esteem these traditions of your fathers which were your glory for centuries."
-Pope Paul VI, Sacrificium Laudis, 1966

“The program for priestly formation is to make provision that the students are not only carefully taught their native language but also that they are well skilled in the Latin language; they are also to have a suitable familiarity with those foreign languages which seem necessary of useful for their own formation or for the exercise of their pastoral ministry.”
-1983 Code of Canon Law, Canon 249