I do not agree that the idea that liturgy should be intelligible is a Protestant idea. If that's the case then all of those compilers and translators of the Roman Missal and Divine Office who produced hand missals and primers over the years must have been cyrpto-Protestants. But I don't think that is the case. Once can simultaneously hold that worship should be intellgible and also that liturgy needn't always be in the vernacular. Father Angelus DeMarco, OFM writes in an article from The American Ecclesiastical Review reposted on CatholicCulture.org:
In liturgical matters the Protestants challenged the use of Latin as a cult-language—which was rooted in theories of a dogmatic nature, which rejected the nature of the Sacrifice of the Mass and the Catholic priesthood. In taking action on this linguistic question it was not so much the Protestant innovation of prescribing a vernacular Liturgy, which interested Trent, as the motive, which led them to this line of action. It is only in the light of the errors, which were then being refuted that the deliberations and decrees can be interpreted. The relevant canon and chapter are: "…If anybody says that the Mass ought not be celebrated except in the common language . . . A.S." (Sess. XXII, c. 9); "Though the Mass contains a great instruction for the faithful people, yet it did not seem expedient to the Fathers that it should be celebrated everywhere in the vulgar tongue..." (Sess. XXII, c. 8). There is neither a dogmatic justification of Latin nor a dogmatic rejection of the use of the vernacular. As far as the vernacular is concerned the Council condemns the mentality, which demands the vernacular as if the nature of the Liturgy made it necessary. The simple use of, or defense of the vernacular or any other language than Latin is not condemned. In establishing reasons why the Church has carefully preserved the Latin language as her liturgical language in the West, Trent did not say that the vernacular is intrinsically evil in itself, nor that its use in the Mass and Sacraments is an impossibility. On the contrary, in clarifying the issue, the Council has left to the Church a criterion for future action, should it ever be necessary to change existing rites and language.
Vernacular liturgy then is not a “protestant idea”; that is something that is frequently said, but the witness of the Eastern Catholic Church, which should be well known, shows it to be false. In the Eastern Church liturgy is celebrated in a variety of vernacular languages such as Greek, Arabic, and Syriac, as well as in “sacred languages” such as Old Church Slavonic and Coptic.
But even in the Latin Church, vernacular liturgy was not unknown, both before the Council of Trent and after. Father DeMarco's article cited above notes several instances where permission for use of vernacular liturgy was given by Rome, including the following:
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
An equally interesting use of vernacular can be read in Claudio Salvucci’s account of “the Tsiatak Nihonon8entsiake, or Book of Seven Nations, published in Montreal in 1865 for the American Indian mission of Lake of Two Mountains, which contained both Mohawk-speaking and Algonquin-speaking Catholics. This mission, like others in the area, was permitted to use the vernacular for the sung propers and ordinaries of the Roman Mass,” on The New Liturgical Movement, which includes images of chants from some of the Requiem Masses in the Mohawk language.
However, even though there is a legitimate place for vernacular languages in liturgical worship, there is also a legitimate place for Latin in the liturgies of Latin Church communities, including the Ordinariates. Even in pre-Ordinariate days, it was (and is) not unusual to find Latin used in the liturgy, especially in the Ordinary chants (Kyrie, Gloria, Credo, Sanctus & Benedictus, and Agnus Dei); see (and hear) for example, this page from St. Clement’s Episcopal Church in Philadelphia.
Pope Paul VI, who presided over the vast extension of vernacular in the liturgy of the Latin Church, recognized the need to preserve and promote the use of Latin as well. In issuing his booklet Jubilate Deo he wrote that his purpose in sending this to all the bishops of the Latin Church was:
to make it easier for Christians to achieve unity and spiritual harmony with their brothers and with the living tradition of the past. Hence it is that those who are trying to improve the quality of congregational singing cannot refuse Gregorian chant the place which is due to it.