t;I wish you good luck..conveys a particular feeling of empathy, a way of expressing how happy you are that they are happy. When good things happen, we show appreciation for them because, one, it’s the polite thing to do (even if you don’t feel particularly excited or attached), . Finding someone to share your life with is an awesome thing. Congratulations to both of you.Congratulations! is just fine. Also, "I wish both of you much happiness together."