Yes, at least one of the two people to be married must be a resident of Spain, although both may be non-citizens of Spain.
Citizens of Spain are Spanish.
Any person swearing allegiance to a country is required to give up allegiance to any other country ! If you become a citizen of a country - you're expected to be faithful exclusively to that country - forsaking all others. For example.... say you were born in Spain - and then became a British citizen... If Spain and the UK were to become enemies - you would be expected to side with the British interests, not Spain - even if that meant joining British armed forces in a war against Spain !
Yes, citizens of Spain can travel to the United States with a valid passport and an approved visa or through the Visa Waiver Program.
Spain is mostly Roman Catholic, but has pockets of Muslims as well. There are diverse existences of other religions, however these are the largest. There is also a growing Atheist population in Spain.
The fact that you hold a US "green card" is essentially irrelevant. What matters is the relationship whatever country you are a citizen of has with Spain. US Citizens, with a US passport, can enter Spain for up to 90 days without a visa. You should check with your country's embassy or consulate to determine whether you can legally enter Spain without a visa.
Queen Mary I married King Philip II of Spain in 1554
National Association of British Schools in Spain was created in 1978.
Legally there is no limit.
It's good if you contact the British High Commission (British Embassy) nearest where you live and ask them. But I think they might ask you to register the marriage at the embassy.
Spain respects the British claim to the islands. It is Argentina which contests the British claim.
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