I agree, I remember an interview where Alexandra D said she had no difficulty in playing her character because it was very natural and similar to herself.
This could mean also, Moodysson was very good at casting actors.
As for dialogues, I got the Australian DVD (the box was destroyed in the long trip but the DVD survived, and amazingly the movie looks worst in its legitimate form than the italian pirate DVIX

) and I see the translaction is almost identical (from swedish to english subtitles, and to italian voice). I was amazed how the dialogue seems natural. And yet I gave a quick look at the script, generally seems they followed it and dialogue was not improvised on the spot. So yes, it's really impressive.
It's a thing I really like in this film, this is one of the very few pieces about adolescent problems that doesnt feel full of crap.