Here is Pope John VIII's response to Archbishop and later St Methodius' request to use Slavonic for the liturgy in Moravia in 879.

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 anywise 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

Realistically speaking, the problems encountered in Moravia by St Methodius actually had more to do with politics than theology. The German bishops resented St Methodius' presence in what they considered to be their own territory, while the Holy See for the most part sided with St Methodius. Their was nothing uniquely anti-Eastern about the German bishops claims, and only a few years earlier, their ruler Louis the German had been allied with Bulgaria where Latin priests were about to be driven out.