Question: If Latin is as important as you said last month, why would we ever use English [the vernacular, be it English, Spanish, or whatever] at Mass?
Answer: The Council of Trent wisely advised that, concerning the Mass, "it has not seemed expedient to the Fathers, that it should be everywhere celebrated in the vulgar tongue" (Session 22, Ch. VIII) and that "If anyone saith ... that the Mass ought to be celebrated in the vulgar tongue only ... let him be anathema" (Session 22, Canon IX). At Our Lady of the Rosary the Mass is always in Latin, with the people reciting or singing the parts appropriate to them in Latin. Yet, implicit in the Council's decree is the possibility that in some times and places it may be expedient to use the vernacular.
Our schedule includes Masses at a rest home where the patients are often close to senility. The Mass is generally attended by a good percentage of non-Catholics (and Catholics who have not heard Mass in Latin for 35 years). At St. Peter's Mission in Miami, most of the parishioners are Bahamins who have always worshipped in English. The vernacular Mass is extremely useful in such situations.
It is also expedient to read in English the longer lessons and Gospels that come occasionally throughout the year. The Easter Vigil, for example, or the Gospel of Palm Sunday (which we are then free to read in its "uncut" version, linking the Last Supper and the Crucifixion).1
Finally, the vernacular has been very well received for the Sacramentals and the Divine Office. Ceremonies like the blessing of holy water, or of first-fruits on the Assumption become quite meaningful. We are also able pray a few elements of the Office as a congregation (Terce following morning Mass, Vespers and Compline on First Friday evenings), which we could not manage otherwise.
Latin is the irreplaceable standard of Western Catholic theology and prayer, but there are times when the vernacular is, as Trent said, "expedient."
1. Matthew 26 & 27, and the parallel Gospels read during Holy Week: Tuesday, Mark 14 & 15; Wednesday, Luke 22 & 23.