This sentence sounds like, "Is it a custom of your country to bring it to a picnic?"
In this Japanese sentence aren't saying "it" though, when the speaker and listner already know what is "it", they'll not say "it" the second time. It's one of the Japanese language habits.
If the speaker want to ask Edo-san that, ”In your country, what do you bring for a picnic?” Japanese sentence will be as follow;
エドさんの国ではピクニックのとき何を持っていきますか。( the point is 何を）