Hi J.
This one is tough. My kids are 3 and 10 months so I'm not there yet but I do have some thoughts on this. My mother did that to me when I was about your daughter's age, maybe a little older and at the time I was super mad at her but in the long run it was for my best. She was able to help me work out some issues I was having and her concern really made me feel like she was someone on my team and that I could go to her with anything I needed help with. It's all in how it's handled in my opinion.
I am an IT person and because of that I know alot about computers and I have every intention of having blocking and monitoring software on my kids computers when that time comes. They will know some of what I have on there and some they will not know. And I won't feel one bit bad about it. If I came across my daughter's journal, I'd probably read it and I won't feel bad about it.
Maybe when we were kids and things weren't so scary out there then you could afford to give your children unlimited privacy but these days? I'm not so sure. Kids are dealing with issues of drugs, sex and violence younger and younger now and even with the best of preparation by their parents are just too young to know how to effectively deal with it. It's our job to be their advocate and to make sure they aren't getting involved in something that's just way over their heads. Too many parents are turning the other way and pretending their blind to their children's actions. It's irresponsible.
Now, would I tell her EVER that I read her journal? No I don't think I would. I think I would find another way to address what I found out without telling her I read her journal. Tell her one of her friends Mom's talked to me or one of her friends etc. And I certainly wouldn't go reading it every day or every month but if my kid is exhibiting strange behavior or hanging out with a different crowd I'll be looking for it then.
This is really all about your personal philosphy on the rights children have to expect a certain amount of privacy. I tend to feel that the safety of my kids trumps their privacy.
I don't think you should address the cursing rant you found, she knows that you wouldn't tolerate that and that's why she wrote it down. I'd actually be proud of her for that. She knows what's acceptable and what isn't and she went the appropriate route to vent her frustrations.
So I don't know if this made you feel better or not but I really wouldn't think twice about it. In my opinion your being a responsible parent who only has her child's best interest at heart. Just don't use typical pre-teen/teen age behavior against her. She's gonna curse you out in that journal and she's going to talk about boys she likes and girls she doesn't like and that's all normal. Only step in when it's dangerous to her physical, moral or spirtual safety.