Imo most vocalists around the world who focus on a style of music that is popular in America are likely to have learned at least enough English to mimic American songs. Because our music industry is so large, any band/singer who covers will most likely have to cover songs written in English.
Regardless, they did require video auditions from all musicians. So I guess, to a certain extent, they did make sure the singers could at least mimic the words of Learn to Fly.