What is acceptable dinner wear for a man and a woman in these countries? Is it more relaxed than France and Italy? Does a man have to wear a tie?
Well, it really depends. If you are going to McDonald's or the equiv, I wouldn't bother. In Split, when we were there last year, I was usually the only one with a jacket. But carry one along, and take it off if necessary. Your question, BTW, is sort of stereotypic, since each country has millions of people and thousands of restaurants. Rules are not that different than in the US - resorty places are less strict than businessy places.
Wear what you want. Nobody cares what you wear, and nobody will notice. Really.