And incidentally, even in Spanish, the gender rule would still apply, since “bravo/a” is first and foremost an adjective, even though it would be used as an interjection in this context. What you are really saying (as you are saying in Italian in the same context) is “You are good,” with the “you are” being understood. That’s why the gender rule would apply even in Spanish, so “brava” would be required when addressing a female in that language, just as it is in Italian.
And incidentally, even in Spanish, the gender rule would still apply, since “bravo/a” is first and foremost an adjective, even though it would be used as an interjection in this context. What you are really saying (as you are saying in Italian in the same context) is “You are good,” with the “you are” being understood. That’s why the gender rule would apply even in Spanish, so “brava” would be required when addressing a female in that language, just as it is in Italian.