No they don't have to be. I think the majority of people using SteemLeo are english speakers, but any content that is high quality and finance-related will do 🦁
I think 20 is too much. We're considering an increase to 15 tags. Maybe 20 in the future, but it feels like a stretch at the moment.
The leo.voter is mostly voting to influence steem rewards, not leo rewards. This could change in the future.