I firmly believe in the importance and necessity of preserving Latin and Gregorian Chant in our traditional Roman Rite. But it is true at the same time that, for weighty reasons, the Popes can grant permission for Mass in the Vernacular. +ABL used to say the TLM in the vernacular is superior to the NOM in the Latin. That is because the TLM is the complete and full Mass, not based only on the language.
Pope Ven. Pius XII also in Mediator Dei says the use of the vernacular in some cases may bring great advantage. But only the Apostolic See, i.e. the Pope can authorize it in those cases.
"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." http://www.vatican.va/content/pius-xii/en/encyclicals/documents/hf_p-xii_enc_20111947_mediator-dei.html
Here's the CE on St. Cyril and St. Methodius, Apostles of the Slavs, who in their Evangelizing Work, celebrated the hitherto Greek Liturgy of the Byzantine Church in the Slavonic tongue of the locals: "Although the Latin holds the chief place among the liturgical languages in which the Mass is celebrated and the praise of God recited in the Divine Offices, yet the Slavonic language comes next to it among the languages widely used throughout the world in the liturgy of the Church. Unlike the Greek or the Latin languages, each of which may be said to be representative of a single rite, it is dedicated to both the Greek and the Roman rites. Its use, however, is far better known throughout Europe as an expression of the Greek Rite; for it is used amongst the various Slavic nationalities of the Byzantine Rite, whether Catholic or Orthodox, and in that form is spread among 115,000,000 people; but it is also used in the Roman Rite along the eastern shores of the Adriatic Sea in Dalmatia and in the lower part of Croatia among the 100,000 Catholics there. Whilst the Greek language is the norm and the original of the Byzantine or Greek Rite, its actual use as a church language is limited to a comparatively small number, reckoning by population. The liturgy and offices of the Byzantine Church were translated from the Greek into what is now Old Slavonic (or Church Slavonic) by Sts. Cyril and Methodius about the year 866 and the period immediately following. St. Cyril is credited with having invented or adapted a special alphabet which now bears his name (Cyrillic) in order to express the sounds of the Slavonic language, as spoken by the Bulgars and Moravians of his day.
Later on St. Methodius translated the entire Bible into Slavonic and his disciples afterwards added other works of the Greek saints and the canon law. These two brother saints always celebrated Mass and administered the sacraments in the Slavonic language. News of their successful missionary work among the pagan Slavs was carried to Rome along with complaints against them for celebrating the rites of the Church in the heathen vernacular. In 868 Saints Cyril and Methodius were summoned to Rome by Nicholas I, but arriving there after his death they were heartily received by his successor Adrian II, who approved of their Slavonic version of the liturgy. St. Cyril died in Rome in 869 and is buried in the Church of San Clemente.
St. Methodius was afterwards consecrated Archbishop of Moravia and Pannonia and returned thither to his missionary work. Later on he was again accused of using the heathen Slavonic language in the celebration of the Mass and in the sacraments. It was a popular idea then, that as there had been three languages, Hebrew, Greek, and Latin, inscribed over our Lord on the cross, it would be sacrilegious to use any other language in the service of the Church. St. Methodius appealed to the pope and in 879 he was again summoned to Rome, before John VIII, who after hearing the matter sanctioned the use of the Slavonic language in the Mass and the offices of the Church, saying among other things:
We rightly praise the Slavonic letters invented by Cyril in which praises to God are set forth, and we order that the glories and deeds of Christ our Lord be told in that same language. Nor is it in any wise opposed to wholesome doctrine and faith to say Mass in that same Slavonic language (Nec sanæ fidei vel doctrinæ aliquid obstat missam in eadem slavonica lingua canere), or to chant the holy gospels or divine lessons from the Old and New Testaments duly translated and interpreted therein, or the other parts of the divine office: for He who created the three principal languages, Hebrew, Greek, and Latin, also made the others for His praise and glory (Boczek, Codex, tom. I, pp. 43-44).