It depends. :)
I believe that in this case, you can either write the comma or not. There is a subtle difference.
'Hezká mladá žena' means 'a pretty young woman.' Pretty is connected to the term 'young woman'. 'Hezká, mladá žena' means 'a pretty (and) young woman.' Both pretty and young are connected to the term 'woman.'
But I wouldn't worry too much about this. Writing commas and capital letters is one of the most difficult parts of Czech orthography and even native Czechs have huge problems with it. :)
Sadly, sometimes you do and sometimes you do not. Depends on what you are trying to say.
Second, international competition - it is a second competition and this time international.
Second international competition - it is the second competition and is international just as the first one was.
In the case of young pretty woman if the comma is not there you are suggesting that if she was old she would not be pretty. If the comma is there then she is both pretty and she is young but the two are not dependent on each other. Honestly though, half of the Czech population gets this wrong..