The thesis that Eastern Orthodoxy is the true religion was turning out to bear the practical corollary that, to share fully and fruitfully in the life of the Body of Christ, one would almost have to become a Greek.
One can only wonder how a triumphalist apologetic that no "true religion" would ever be slow to implement the local vernacular (even eg. in a local parish initially established for immigrants in a foreign country who don't yet know the local language) understand historical instances of not only Roman Catholic slowness, but direct opposition to Liturgy in the vernacular during periods when Orthodoxy championed it.
Here is one instructive example, of Sts. Cyril and Methodius, from Bishop Kallistos Ware, The Orthodox Church (full text online):
"One cannot overestimate the significance, for the future of Orthodoxy, of the Slavonic translations which Cyril and Methodius carried with them as they left Byzantium for the unknown north. Few events have been so important in the missionary history of the Church. From the start the Slav Christians enjoyed a precious privilege, such as none of the peoples of western Europe shared at this time: they heard the Gospel and the services of the Church in a tongue which they could understand. Unlike the Church of Rome in the west with its insistence on Latin, the Orthodox Church has never been rigid in the matter of languages; its normal policy is to hold services in the language of the people.
"In Moravia, as in Bulgaria, the Greek mission soon clashed with German missionaries at work in the same area. The two missions not only depended on different Patriarchates, but worked on different principles. Cyril and Methodius used Slavonic in their services, the Germans Latin; Cyril and Methodius recited the Creed in its original form, the Germans inserted the filioque. To free his mission from German interference, Cyril decided to place it under the immediate protection of the Pope. Cyril’s action in appealing to Rome shows that he did not take the quarrel between Photius and Nicholas too seriously; for him east and west were still united as one Church, and it was not a matter of primary importance whether he depended on Constantinople or Rome, so long as he could continue to use Slavonic in Church services. The brothers traveled to Rome in person in 868 and were entirely successful in the appeal. Hadrian II, Nicholas I’s successor at Rome, received them favorably and gave full support to the Greek mission, confirming the use of Slavonic as the liturgical language of Moravia. He approved the brothers’ translations, and laid copies of their Slavonic service books on the altars of the principal churches in the city.
"Cyril died at Rome (869), but Methodius returned to Moravia. Sad to say, the Germans ignored the Pope’s decision and obstructed Methodius in every possible way, even putting him in prison for more than a year. When Methodius died in 885, the Germans expelled his followers from the country, selling a number of them into slavery. Traces of the Slavonic mission lingered on in Moravia for two centuries more, but were eventually eradicated; and Christianity in its western form, with Latin culture and the Latin language (and of course the filioque), became universal. The attempt to found a Slavonic national Church in Moravia came to nothing. The work of Cyril and Methodius, so it seemed, had ended in failure.
"Yet in fact this was not so. Other countries, where the brothers had not themselves preached, benefited from their work, most notably Bulgaria, Serbia, and Russia. Boris, Khan of Bulgaria, as we have seen, wavered for a time between east and west, but finally accepted the jurisdiction of Constantinople. The Byzantine missionaries in Bulgaria, however, lacking the vision of Cyril and Methodius, at first used Greek in Church services, a language as unintelligible as Latin to the ordinary Bulgar. But after their expulsion from Moravia, the disciples of Methodius turned naturally to Bulgaria, and here introduced the principles employed in the Moravian mission. Greek was replaced by Slavonic, and the Christian culture of Byzantium was presented to the Bulgars in a Slavonic form which they could assimilate. The Bulgarian Church grew rapidly. Around 926, during the reign of Tsar Symeon the Great (reigned 893-927), an independent Bulgarian Patriarchate was created, and this was recognized by the Patriarchate of Constantinople in 927. The dream of Boris — an autocephalous Church of his own — became a reality within half a century of his death. Bulgaria was the first national Church of the Slavs.
"Byzantine missionaries went likewise to Serbia, which accepted Christianity in the second half of the ninth century, around 867-874. Serbia also lay on the dividing line between eastern and western Christendom, but after a period of uncertainty it followed the example of Bulgaria, not of Moravia, and came under Constantinople. Here too the Slavonic service books were introduced and a Slavonic-Byzantine culture grew up. The Serbian Church gained a partial independence under Saint Sava (1176-1235), the greatest of Serbian national saints, who in 1219 was consecrated at Nicaea as Archbishop of Serbia. In 1346 a Serbian Patriarchate was created, which was recognized by the Church of Constantinople in 1375.
"The conversion of Russia was also due indirectly to the work of Cyril and Methodius; but of this we shall speak further in the next section. With Bulgars, Serbs, and Russians as their "spiritual children," the two Greeks from Thessalonica abundantly deserve their title, "Apostles of the Slavs."http://www.fatheralexander.org/booklets/english/history_timothy_ware_1.htm