Feast of Sts. Cyril and Methodius
It is once again instructive to realize that they first didn't first translate the bible into the new language. They translated the liturgy, meaning the Mass. Why didn't they just start with Scripture? The likely answer is that the best way in the 9th century as in every century is to present the gospel in the context of the Church. In all likelihood, the literacy rate in the 9th century Slavic nations was very low, and the best way to preach the gospel was in the liturgy which is literally filled with scripture. In the end of his life, one of the brothers indeed did translate the entire bible as well as many writings of the Church Fathers.