Doesn't השמלה שלי mean "my dress?". I thougt that השמלה היא שלי would be a better translation, am I wrong?
I don't think that השמלה היא שלי is incorrect, but it's definitely unnatural. The distinction between these two meanings is mostly made by the stress (and by context): השמלה שלי - 'the dress is mine' ; השמלה שלי - 'my dress'.
So "my dress" and "the dress is mine" are translated in the same way in Hebrew?
Yes, I thought this should be hasimla zeh sheli