Gosh, these are such innocent yet real questions for her. She does not know the difference between good and bad language. It doesn't matter where she learns this stuff, you just need to keep teaching her and role modeling and educating her what is right and what is not. She's going to be exposed to many undesirable circumstances in her life that you have no control over.
And this part about gently correcting her will never end....it's very hard to just turn it off one day, as is my experience with my older (22) daughter. I'm still telling what I believe to be true and beneficial.
Now, my oldest daughter was raised by a magnificent step dad who also came from a strict home, where as I did not. And it is important to understand that the human bonding process is a long and fragile one. There will be many joys and many disappointments. The biggest disappointment after 13 years of raising her, financially and psychologically supporting her, she up and left for her father's at the age of 15. It mattered not that we all knew what a loser he was. SHE had to determine that on her own. SHE had to discover that for herself. It was her journey. Not mine to command.
I state over and over on this site, that it does not matter what a great step parent is substituted in for the child. All children have a biological need starting in their teens to understand themselves, and this insight is usually gained by knowing their genes, meaning parent they are separated from.
Happy parenting!
***EDIT***
I had a personal exchange with this sweet mama...and wrote the following note to her. And then realized my private message was more apropros:
Well, you certainly made an old mama proud to be able to help and add some life long perspective.
I did not read any of the other answers to your question. Your scenario brought instant flash back to me though.
To this day I can remember asking my mom what the middle finger meant, as I had seen the boys in the neighborhood pointing it at each other and I could tell there was more to it. She slapped me in the face. I was beyond shocked and mortified by such a harsh response. I really wanted to know. She never did answer my question and the but what she did teach me was NEVER to go to her and ask for anything borderline. No mutual trust and respect in that relationship. So I understand how parents can momentarily loose their perspective. We all do, especially being a mom.
And the biggest insight into parenting I ever had was from my very own daughter. I was mad over something long forgotten now, sent her to her room, I went to mine to cool off and she gently approached me and said, "Mommy, I'm still learning. Please don't be so mad at me." I cry as I type this...such sweet innocence.