A good question.  I believe that all or nearly all conservative scholars will agree that the Pentateuch was originally written down in Hebrew.   Consider the name given to God in Exodus  YHWH. This name can only have ever been Hebrew.  This only makes sense if it was given to Moses in Hebrew.  There is literally no evidence for an earlier version of the Jewish Bible in a language other than Hebrew.  Unfortunately, for many of the OT books, we do not know for sure the original date of the writing.  However, all of our earliest evidence is for it having been written in Hebrew, and there is literally no evidence for it having been written previously in another language.  Therefore, the most reasonable conclusion is that the original language is Hebrew.  Those who claim differently are arguing from a position literally without evidence.  This is a weak position to argue from.

But, folks can criticize this understanding with some reason.  It is not a crazy idea to criticize the belief that Moses wrote in Hebrew around 1500 BC for the simple reason that we do not have any physical evidence in the form of inscriptions in Hebrew from this early.  The argument for a non-Hebrew original comes from a lack of evidence to support the original being Hebrew, not from any actual evidence to the contrary.  The earliest evidence for written proto-Hebrew is from the twelfth of thirteenth century BC.  This does not mean that Hebrew did not exist before this time.  It is a lower limit for the age of the language.   Some say that the Hebrew language did not even exist in the fifteenth century BC, but, of course, they do not have any evidence that the language did not exist.  Some say that Moses, if he even lived at all, was certainly illiterate.  But they have no evidence for this as well.

I conclude that the original of the Old Testament was in Hebrew, but will admit that this is a reasonable conclusion, but not proved.

John Oakes