47 Where, according to old or immemorial custom, some popular hymns are sung in the language of the people after the sacred words of the liturgy have been sung in Latin during the solemn Eucharistic sacrifice, local Ordinaries can allow this to be done "if, in the light of the circumstances of the locality and the people, they believe that (custom) cannot prudently be removed." The law by which it is forbidden to sing the liturgical words themselves in the language of the people remains in force, according to what has been said.
Source: Musicae Sacrae (Vatican.va)