Let's say that the question is like this one. It doesn't specify a jurisdiction, though it recommends the United Kingdom. If Hypothetical Answer A talks about, say, Germany, and you have some information about Italy, then perhaps a new answer would be warranted - if the laws are different enough. If the laws themselves are almost identical, then a comment could be just enough. However, if there are analogous laws in both countries, but they have different implications, then I think a new answer is warranted.
There's always that grey area of subjectivity, but most of the time, you should be able to figure out what the similarities warrant.
Both a comment and an answer can be helpful. But if you have a lot of new information because the law is so different, then an answer is probably warranted. There's nothing inherently wrong about just posting a comment, though - you're contributing helpful information.
You can also ask about a specific case in chat.