How do I know which country a question is from (different laws in different countries)?

The person asking the question has the responsibility to tell you what their question is, and if this includes what country's laws they're asking about, they need to tell you that too.

Yahoo can't tell what country a person is even in, because they could be in, say, the USA, but using a proxy server in, say, Japan, and it would look to Yahoo like their "signal" was coming from Japan.