A pretty solid note on the occasions when Latin was set aside in favor of the vernacular, at least for a time:
"Although Latin prevails in the West as a unified liturgical language, in the face of certain circumstances the Roman church has made exceptions to provide a language in the Liturgy more familiar to the people. It is in the ninth century among the Slavic nations that we find a departure from liturgical Latin in divine worship. A privilege was first granted to Sts. Cyril and Methodius, by Pope Hadrian II in 869, and again by Pope John VIII in 880 to use the vernacular in the Liturgy.24 It was in practice in the present-day territories of Czechoslovakia; afterwards it was introduced by way of legitimate custom into the regions of Croatia. In the course of the years the Holy See has been quite positive in declaring her mind not only by the decrees of the Popes, especially Urban VIII, Innocent X, Benedict XIV, Pius VI and Leo XII, but also by compiling and publishing liturgical books in Glagolithic (Old Slavonic) characters. Among the most important pontifical documents for the use of this privilege is the rescript of Pope Innocent IV granted in 1248 to Philip, Bishop of Senj.25 Today members of the Roman Rite celebrate the Liturgy in the paleo-slavic language in the Croatian diocese of Senj, Modrus, and Kirk, and in some parishes of the dioceses of Sibenik and Split (present-day Yugoslavia), and in those places where there are large numbers of the Slavic races.
Another example of the flexibility of which the Roman rite is capable is the privilege granted for the use of Chinese as a liturgical language. History records in the fourteenth century that the first Franciscan missionary to China, John of Monte Corvino, used the vernacular in the Liturgy.26 Pope Paul V, in a brief of June 27, 1615, granted the same privilege to Jesuit missionaries.27 As recently as 1949, the privilege to use the Chinese literary language in the Liturgy was granted by the Holy Office.28 When conditions return to normal in China, and when Rome finally has approved a completed Chinese-Latin missal, this decree will take effect in all parts of that country."
Read the full article HERE