Two mainly: The Old Testament was originally Hebrew with a bit of Aramaic (a sister language to Hebrew) in a bit of Ezra and a portion of Daniel. The New Testament was originally written in Greek.
Over time, it was translated over and over again by differeng generations. From the Septuagint (OT translated from Hebrew to Greek) to Latin (during the time when the Roman Empire was in power), so on, so forth, until we find it in countless languages today.
For english translations, it is recommended to use several good translations. For instance, the difference between KJV and NIV is that KJV gets very literal (He saw a house and the house was red) while the NIV is conceptual (He saw a red house). Yet at times, the KJV is more expressive in sheer style compared to the efficient NIV (c.f. the Psalms, which the KJV better expresses poetically rather than the effecient NIV).
NIV and NKJV would be good side by side for those who want to go a bit deeper than the surface.