60 The use of the Latin language, customary in a considerable portion of the Church, is a manifest and beautiful sign of unity, as well as an effective antidote for any corruption of doctrinal truth. In spite of this, the use of the mother tongue in connection with several of the rites may be of much advantage to the people. But the Apostolic See alone is empowered to grant this permission. It is forbidden, therefore, to take any action whatever of this nature without having requested and obtained such consent, since the sacred liturgy, as We have said, is entirely subject to the discretion and approval of the Holy See.
Source: Mediator Dei (Vatican.va)