What is the Hebrew name for the Yiddish name Leib? My daughter wants to name her upcoming baby after my father whose name was Leo/Leib but we dont know for sure what his Hebrew name was

It is actually very commendable to translate Yiddish names to Hebrew, which is the “Holy Tongue”, and the original Jewish language used by all of our fore-fathers in the Tanach (Bible) going back to Adam and Eve (Hebrew, not Yiddish, names!). In addition, the odds are higher that he’ll be proud to use his Jewish name if it’s in Hebrew rather than Yiddish (considered old-fashion). In all probability your father’s Hebrew name was Aryeh, which was a common “pair”: Aryeh-Leib, which is Hebrew and its Yiddish translation for lion (which was often translated to European languages as Leo or Leon). There was a similar, though less common, Hebrew-Yiddish “pair”, Yehuda-Leib whose origin is the fact that the flag of the Biblical tribe of Judah (Yehuda) was the lion. If you can’t verify it [maybe try checking a family tree to see if there are a lot of Aryehs or Yehudas in the family], in all probability the name was Aryeh who's appearance with Leib was much more common and is a more exact translation. If you prefer, some favor a more modern version like Ari, which has an identical meaning. If you’d like to read more on the topic, in my forthcoming book, L’Harim et HaDegel (“Raise the Flag”), about the Israeli Flag and speaking Hebrew, there is a chapter on the importance of Hebrew names. All the best, Rav Ari Shvat