The Old Testament was written in the Hebrew language primarily for the Jews. The first five books were written by Moses after the Exodus from Egypt. This was also the establishment of Judaism (which is the Old Covenant, or Testament with Israel). The last of the Old Testament was written about 444 BC.
The New Testament (which is what the Old Testament was prophesying toward) was written in the Greek (which due to Alexander's conquests was the prevailing language over all the known world). This was to establish the church (and Christianity).