Those masses are usually held by smaller congregations which reject the 2nd vatican council. The reasons are religious in nature. It's not because the laity understands Latin. I assume the clergy would understand what they are saying.
Yes, quite a few of the people who attend the Tridentine Mass do so at chapels or churches run by the Society of St. Pius X, or the Society of St. Pius V whicgh are groups which reject the validity of Vatican II and, somewhat ironically, end up rejecting the authority of the pope in favor of their own judgements about doctrinal and non-doctrinal matters. A growing number of Catholics are availing themselves of validly celebrated Masses in the extraordinary form (the pre-Vatican II Latin Mass) through groups which are in full communion with the Church such as the Priestly Fraternity of St. Peter.
That all being said, the Catholics I have met who attend the older rite of the Mass all have at least a passing understanding of ecclesial Latin. And my parents have both told me it was uch the same back in the 50's. Go figure. As I said before, it's not rocket surgery to open up a Latin-English Missal :lol:
The whole criticism of people not understanding anything during the Mass back in the day is mostly a bogey man conjured up by people who should've known better. One doesn't need to be a latinist to understand the ordinary of the Mass (those parts that remain the same in every liturgy). Heck, I could prayerfully participate in prayers such as the Agnus Dei, the Credo, the Pater Noster, the Sanctur, the Gloria, and the Kyrie (in Greek) before I ever took a single class in Latin. As for the propers, you can follow along in the Missal for those and still have a very prayerful experience.
The people sitting in the pews today listening to Mass in English are, in general, woefully catechized and have little understanding of what's actually occuring when the priest confects the Eucharist. Celebrating the Mass in English has a less than stellar record when it comes to the people actually understanding the faith. In a religion which holds to
lex orandi, lex credendi that is a
huge probelm.
Pax,
Chris