For whatever it’s worth, in the world of opera, it’s “In bocca al lupo,”—“In the mouth of the wolf!”
And the stock reply is crepi il lupo!, ‘may the wolf die’.
And dancers often use merde.
I heard another origin for “break a leg”
Artists entry used to be on the theater side, on the small street where horse powered vehicles and drivers would wait after bringing their masters to performance.
So many spectators would mean many carriages and lot of horse shit there. And of course increased risk for artist to slip on it and break a leg.