And in Yiddish it changed to "Mazl tov". What happened to the Yiddish incubator by the way?
I think that in Hebrew, too, the original intention was future-looking. They say מזל טוב in weddings and child births etc., wishing you good luck in the future. Then, since it got attached to happy occasions, they started using it for happy occasions even if they only celebrate recent-past achievements.