While the Psalms were all written in Hebrew, neither the Eastern nor the Western Church used them liturgically in their original language. The Eastern Church chanted them in Greek (and later a proto-version of Russian) while the Western Church chanted them in Latin. Numerous parts of the Western liturgy derived their names from their first words in Latin. Hence, the Lord's Prayer became known as the Pater Noster because these are its first two words in Latin. Jesus didn't pray that prayer originally in Latin, either, but there you are.