Very unbelievable plot (spoilers)
I hate when characters in movies act overtly stupid or unrealistic and this is the case with this movie.
First, Anna is supposedly a typical naive young woman. The movie mentions that she understands that the dead girl is likely an underage prostitute, i.e. sex with her is an act of statutory rape. The girl's diary can help police establish people who pimped her or had sex with her. Nevertheless, she deliberately steals the diary withholding this important evidence from police, committing a criminal act. The way she's portrayed in the movie doesn't imply she would commit this crime.
Second, Anna finds a business card in the diary. She approaches the owner of the restaurant that appears on the card and asks him to to translate the diary. It's very stupid and unrealistic. Any person with the slightest intellect would understand that this business is very likely somehow responsible for pimping the girl.
And third, when Anna reproached mafiozi for what they have been doing, asked them if they're not ashamed, that was just lame. I don't believe real people can be so dumb. If she were a real person, she would qualify for Darwin award (probably posthumously).
There are a few other stupid moments, mostly about Anna. I actually found her character very annoying and wished she died in the movie.
The rest of the movie was OK and I enjoyed the fight scene, one of the best I've seen in a Hollywood movie.