I’ve always wondered about this translation. The Hebrew says וַיֶּחֱזַק לֵב-פַּרְעֹה, “Pharaoh’s heart was strengthened.” Elsewhere it says וְהַכְבֵּד אֶת-לִבּוֹ, “He weighted (made heavy) his heart.” I don’t know where “hardened” comes from.

Chag sameach!