10 The spiritual gift which priests receive at their ordination prepared them not for a sort of limited and narrow mission but for the widest possible and universal mission of salvation "even to the ends of the earth" (Acts 1:8), for every priestly ministry shares in the universality of the mission entrusted by Christ to his apostles. The priesthood of Christ, in which all priests really share, is necessarily intended for all peoples and all times, and it knows no limits of blood, nationality or time, since it is already mysteriously prefigured in the person of Melchisedech. Let priests remember, therefore, that the care of all churches must be their intimate concern. Hence, priests of such dioceses as are rich in vocations should show themselves willing and ready, with the permission of their own ordinaries (bishops), to volunteer for work in other regions, missions or endeavors which are poor in numbers of clergy. Present norms of incardination and excardination should be so revised that, while this ancient institution still remains intact, they will better correspond to today's pastoral needs. Where a real apostolic spirit requires it, not only should a better distribution of priests be brought about but there should also be favored such particular pastoral works as are necessary in any region or nation anywhere on earth. To accomplish this purpose there should be set up international seminaries, special personal dioceses or prelatures (vicariates), and so forth, by means of which, according to their particular statutes and always saving the right of bishops, priests may be trained and incardinated for the good of the whole Church. Priests should not be sent singly to a new field of labor, especially to one where they are not completely familiar with the language and customs; rather, after the example of the disciples of Christ, they should be sent two or three together so that they may be mutually helpful to one another. Likewise, thoughtful care should be given to their spiritual life as well as their mental and bodily welfare; and, so far as is possible, the circumstances and conditions of labor should be adapted to individual needs and capabilities. At the same time it will be quite advantageous if those priests who go to work in a nation new to them not only know well the language of that place but also the psychological and social milieu peculiar to the people they go to serve, so that they may communicate with them easily, thus following the example of Paul the Apostle who could say of himself: "For when I was free of all I made myself the servant of all, that I might win over many. Among Jews I was a Jew that I might win over the Jews" (1 Cor 9:19-20).
Source: Presbyterorum Ordinis (Vatican.va)