She is the youngest and her sisters do not talk like that? You and your wife do not talk like that? (Be honest…not even when you are very angry, right?) If you use that kind of language, it is time to talk to your daughter and agree that it does not become either one of you and that you both need to stop together. Just as you can appreciate when hearing her how bad it is, she can imagine listening to you and realize that foul language doesn’t make any points with anybody. Some tolerate any kind of language, that is true, but no one really thinks more highly of those who talk like that.
I think that you may want to talk to her about the possibility that she may have picked up your temper. At any rate, ask her what she would think if you or her mother were to talk as she has been talking. Do respected people talk like that? Unfortunately, sometimes they do. (Never lie to your children–they are not stupid.) Do they lose respect when they lose their cool and use that kind of talk where they can be overheard? Yes, they do. It is the language that signals a loss of self-control. It is also a kind of language that easily routes its way through the part of the brain that blurts things out. (People familiar with stroke victims can tell you that language does have different routes, such that people might be able to speak clearly via the “blurt it out” route but can only speak jibberish through the route they control.)
Yes, it is reasonable to point out to her that she has to have control of her temper, her mouth, and her actions before she can have an i-Phone. The things you text are there forever. Someone can always find them. If you can’t think before you talk or act where someone can hear you and see you in person, you won’t think before you text or talk on a phone or when someone is capturing on video. If you don’t think before you text or talk on a phone, you are going to say something regrettable that you cannot banish from what can easily become the public record.
The new lesson of the 21st century is that everyone can hear you all of the time, can film you anywhere and everywhere, and can publish what you said and did in ways that you cannot control and can never get back. There has never been a time in history when those who lose their cool will be at a bigger disadvantage than they will be in our current technological climate. If your daughter has one outburst like she just had as an adult, she could ruin her career. It is good that you and your wife are on the same page on this one. Stick to your guns.
In the meantime, spend a lot of time with your daughters, but especially that 12 year old. Spend time in ways that you will both enjoy and spend time in which you learn about her particular gifts and view of the world. Draw out the best in her and openly appreciate the best in her. You and your wife need to have your “emotional bank account” full with your girls, but especially with that one.