21 Moreover it is necessary that those who are called to this kind of apostolate should not only get the spiritual and intellectual training that befits ecclesiastical students, before going out on the mission field, but should learn in addition those subjects which will be most useful to them when they come to preach the Gospel in foreign lands. Hence they should be given a sound knowledge of languages, especially of those which they will require at some future date. Besides, they should be sufficiently instructed in the sciences of medicine, agriculture, ethnography, history, geography, etc.
Source: Evangelii Praecones (Vatican.va)