付了禮金之後半年也沒回禮, 可否民事索償?

The answer is yes. However, it may have some difficulties.

1. It is usually a breach of contract case, given that the cash given out is a condition of the marriage, rather than a gift.

2. You must be able to prove it is customary that return gift is given (i.e. there is a reasonable expectation that a return gift will be available).