I've always been taught, and have always accepted, that the original languages in which the Bible was written was Hebrew and Aramaic for the Tanahk (the "Old Testament") and Koine Greek for the New Testament.
But I just read a blurb that is a new thought for me:
1. We don't have the original manuscripts of the Bible writings; we have copies of copies for the most part, and
2. Even if we had the originals, how would we know they were the originals? (Perhaps if it was in a box with a note inside that said "This is the original letter that Paul sent to the Ephesians" we'd have confidence that we had an original.)
What this means is that we don't know what language the original Bible documents were written in.
Moses was probably brought up as a bi-lingual baby, speaking both Egyptian and Hebrew. However, by the time he reached school-age, he was probably speaking Egyptian almost exclusively, making that his native tongue. So it might have been natural for him to write the Pentateuch in his native Egyptian. On the other hand, writing for his fellow Hebrews, he might have written in their native language (assuming it was Hebrew and not Egyptian). This latter case might also be bolstered by the idea that he spent forty years with his wife's family outside of Egypt, so whatever language they spoke might have become his native language by the time of the Exodus.
The conversations in the New Testament were probably not conducted in Greek for the most part, but rather in Aramaic, at least the ones in and around Israel. So when telling the story of Jesus, especially to fellow Israelites, it might make sense to write it in Aramaic originally.
But the short version is simply that we don't know.
Having thought about the issue for maybe a grand total of 30 minutes, I still tend to think that the best case can be made for the original documents to have been written in Hebrew, Aramaic, and Koine Greek, as I've been taught all my life. But the fact is ...
... we just don't know.