Tomb of Pope John XXIII via Diana at the German language Wikipedia.

Listers, please take the time to review these quotes on the importance and immutability of Latin in the Roman Catholic Church. Note the sources: many of which can certainly not be labeled (and discarded) as “traditionalists.” Latin in the Church is not a liberal or conservative issue, but a Catholic one.1
It [the Traditional Latin Mass] is virtually unchanged since the third century.
John Henry Cardinal Newman, “Callistus”

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 that is universal, immutable, and non-vernacular.
Pope Pius XI, Officiorum Omnium, 1922

The day the Church abandons her universal tongue [Latin] is the day before she returns to the catacombs.
Pope Pius XII

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 corruptions of true doctrine.
Pope Pius XII, Mediator Dei, 1947, Sec. 60

Latin is the immutable language of the Western Church.
Pope John XXIII

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.
Pope John XXIII, Veterum Sapientia, February 22, 1962 (just eight months before the opening of Vatican II), chap. 13

We also, impelled by the weightiest of reasons … 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 some 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.
Pope John XXIII, Veterum Sapientia,
February 22, 1962 (just eight months before the opening of Vatican II), chap. 13

The use of the Latin language … is to be preserved in the Latin rites.
Second Vatican Council, Sacrosanctum Concilium
(Constitution on the Sacred Liturgy), para. 36.1

In accordance with the age-old tradition of the Latin rite, the Latin language is to be retained by clerics in the Divine Office.
Second Vatican Council, Sacrosanctum Concilium
(Constitution on the Sacred Liturgy), para. 101.1

If the Church is to remain truly the Catholic Church, it is essential to keep a universal tongue.
Cardinal Heenan (1967)

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, August 15, 1966, Epistle to Superiors General of Clerical Religious Institutes Bound to Choir, on the Celebration of the Divine Office in Latin

We cannot permit something that could be the cause of your own downfall, that could be the source of serious loss to you, and that surely would afflict the Church of God with sickness and sadness…. The same Church gives you the mandate to safeguard the traditional dignity, beauty, and gravity of the choral office in both its language [Latin] and its chant…. Obey the commands that a great love for your own ancient observances itself suggests….
Pope Paul VI, Sacrificium Laudis, August 15, 1966,
Epistle to Superiors General of Clerical Religious Institutes Bound to Choir, on the Celebration of the Divine Office in Latin

We address especially the young people: In an epoch when in some areas, as you know, the Latin language and the human values are less appreciated, you must joyfully accept the patrimony of the language which the Church holds in high esteem and must, with energy, make it fruitful. The well-known words of Cicero, “It is not so much excellent to know Latin, as it is a shame not to know it” [Non tam praeclarum est scire Latine, quam turpe nescire (Brutus, xxxvii.140)] in a certain sense are directed to you. We exhort you all to lift up high the torch of Latin which is even today a bond of unity among peoples of all nations.
Pope John Paul II, 1978

Nevertheless, there are also those people who, having been educated on the basis of the old liturgy in Latin, experience the lack of this “one language,” which in all the world was an expression of the unity of the Church and through its dignified character elicited a profound sense of the Eucharistic Mystery. It is therefore necessary to show not only understanding but also full respect towards these sentiments and desires. As far as possible these sentiments and desires are to be accommodated, as is moreover provided for in the new dispositions. The Roman Church has special obligations towards Latin, the splendid language of ancient Rome, and she must manifest them whenever the occasion presents itself.
Pope John Paul II, Dominicae Cenae,
February 24, 1980, sec. 10
  1. Thanks to John Henry for compiling the quotes [↩]