score:4
He's probably referring to John XXIII's 1962 encyclical Veterum Sapientia: On the Promotion of the Study of Latin:
In accordance with numerous previous instructions, the major sacred sciences shall be taught in Latin […] professors of these sciences in universities or seminaries are required to speak Latin and to make use of textbooks written in Latin.
The study of Latin is still required by the current canon law:
Can. 249 The program of priestly formation is to provide that students not only are carefully taught their native language but also understand Latin well and have a suitable understanding of those foreign languages which seem necessary or useful for their formation or for the exercise of pastoral ministry.