Historically what percentage of the Catholic Church's liturgical function was done in Latin?
It seems logical to assert that the majority of the Roman Rite Masses were entirely in Latin, except for the sermons which would naturally be in the vernacular. Even this was sometimes done in Latin, especially in universities or where the majority of the mass attendees understood Latin.
Historically, some papal Masses and funerals of Roman Cardinals the Gospel would be chanted in Greek as a sign of the universality of the Catholic Church. See this article: The Greek Deacon of the Papal Rite of Mass.
St. Francis de Sales preached eloquent sermons in French around the time of Protestant Revolution. Missionaries would have a hard time making converts if the sermons at mass were habitually in Latin.
Another example is that of Abbot Aelfric:
In the final decade before the year 1000, an Anglo-Saxon abbot named Aelfric wrote and distributed three series of homilies in the Old English language, all, or nearly all, of which have survived. David Knowles, in his monumental volume, The Monastic Order in England, describes Aelfric’s place in early English church history as “second only to Bede and in direct spiritual descent from him.” Aelfric appears to clearly state his purpose for the composition of his homilies in two prefaces attached to the first series of forty homilies, with the first preface written in Latin and the second in Old English. These homilies, he states, have been compiled and translated from the works of church fathers such as Augustine, Jerome, and Bede, and are to be read by priests in English churches to the unlearned, who cannot understand the Latin of the Scriptures and liturgy, but only their own native tongue. - [“In the Language to which They Were Born”: A Study of Audience for the Vernacular Catholic Homilies of Aelfric](“In the Language to which They Were Born”: A Study of Audience for the Vernacular Catholic Homilies of Aelfric)
Hymns and other pieces of sung liturgical songs or psalms were uniquely Latin in Latin Masses of the Roman Rite. In fact the vernacular practice of doing such creeped in after the Protestant Revolution.
Singing in the vernacular may not be substituted for the latter. This abuse crept in after the Reformation, and flourished in the eighteenth century, particularly in Germany and adjacent countries. The wish of the Church is that this abuse should be everywhere extirpated, while violence to local customs be avoided. - Singing by the people
This said, not all Masses in the Roman Rite were in Latin.
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 (Slavonic) in the Liturgy.
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. As recently as 1949, the privilege to use the Chinese literary language in the Liturgy was granted by the Holy Office.
Still further concessions have been granted:
a) During the fourteenth century the Roman Liturgy in its Dominican variant was translated into Greek for use by the Dominican missionaries in Greece.
b) Permission had been granted to celebrate the Dominican Liturgy in the Armenian classical language in Armenia.
c) At the end of the sixteenth century missionaries of India of the Latin rite were allowed to celebrate Mass in Syriac.
I) In 1959, the Holy See renewed Germany's privilege to use the vernacular (German) in the Epistle and Gospel after they are recited in Latin. - Liturgical Languages
In other words, the exact same Mass could be said in at least three languages: Latin, Greek and Old Slavonic and thus were considered Roman Catholic and not Eastern Rite Catholics. Please note that some authors employ the term Greek Rite or Old Slavonic Rite, but in this case, the term means a variation or usage of the Roman Rite.
The Roman Rite is used in Dalmatia in an Old Slavonic version (written in Glagolitic letters), occasionally in Greek in Italy; but in any language it is always the Roman Rite. - Catholic Encyclopedia