They did not speak Hebrew in early Genesis ... the best reasonable guess would be a language similar to Aramaic. The Jewish people were in Egypt from the time of Joseph until the time of Moses ... and the language they most likely spoke there was Egyptian. The development of the Hebrew language would have been post-Exodus, and Babel was early Genesis.
Those Jews who had stayed around Israel running the place for the conquerors while the captives were away, spoke Aramaic, which was the common language of the place. After the return, Hebrew became first a sort of liturgical, then scholarly language, necessary in the Temple.
The OT was written in various tongues, Hebrew among them, but was translated into Greek by Jewish Scholars who needed to get it into the hands of Hellenic Jews all over the Mediterranean who no longer spoke their native languages.