In English, speeches always ignore the 'dear' bit.
For example, in public speaking competitions (many, many years ago!) I learnt to start with something along the lines of "Mr Mayor, Ladies and Gentlemen". When I was best man at a friend's wedding I started with "Stuart and Judith, Ladies and Gentlemen".
I'd recommend something similar in your case. i.e. address the bride and groom by name and then just use 'ladies and gentlemen'. If you (or the person you're translating for) insists upon using a title for the guests, then, in English, it's not 'dear guests' but 'honoured guests' [however I'd strongly advise against using such an archaic form!].
Chris