The picture of the missions on the site says 'any global league' so I imagine it works for LAN people watching NA, or NA watching EU, etc.
Although it may only work for watching the regions listed in the article which are Brazil, Turkey, Latin Amer, EU, NA, Oceania, and Japan. This would mean no rewards for watching Korea or China which is real upsetting since i watch mostly NA and Korea.
While we do not have missions support in all leagues yet (which is what that part of the article is referring to), any pro level games on watch.lolesports.com like NA and KR can be used to earn rewards if you have the missions.