The first Christian Bible was in Greek. The New Testament books were in Greek and the Septuagint, a Greek translation of the Tanakh, was used as the Old Testament. The next translation of the whole Bible into one language was done in Latin, the Vulgate. The Latin translator, Jerome, looked at both Hebrew and Greek Old Testaments for the translation. Only much later was the Bible translated into modern languages.
Both Covenant and Testament are English translations of the Latin "Testamentum" which means covenant. I think Old Testament was probably used first in English. Certain groups of Christians preferred to use Old Covenant, but it is rarely used today. Both Old Testament and Old Covenant mean the same thing, the Tanakh. We have a special term for the five books of Moses, the Pentateuch. Penta means five in Greek. The word Pentateuch is not used much except in religious discussions.
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