Priests of Opus Dei say the Mass according to the new rite that came out after Vatican II (the Novus Ordo). When they celebrate Mass in a center of Opus Dei they often say it in Latin, but sometimes it’s in the vernacular.
Those who find this practice objectionable should note that Vatican II, while allowing the vernacular to be introduced into the Liturgy, actually encouraged the continued use of Latin. See Sacrosanctum Concilium for more information.
Aside from the liturgical reasons for saying the Mass in Latin, there is the practical reason that centers of Opus Dei tend to have people living in them and visiting from many different countries. Having the common language of Latin used at Mass allows those attending to participate more readily. Mass is usually said in the vernacular during retreats and other public functions.
There’s also the symbolic reason that Opus Dei, being spread throughout the world, and having a universal mission, more readily expresses that reality by the use of Latin.
Having attended Mass in Latin while on my “annual course” (see Question 18), and relying on the fact that many English and Spanish words have their roots in Latin, I’ve learned a lot about the true meaning of many of the words and phrases in the English translation of the Mass. The experience has definitely improved my understanding of the Mass.