Studies have shown that a language and culture can be a major issue in air accidents. What does the pilot really mean? Is the polite Korean co-pilot going to be able to properly communicate to the gruff New York tower that they need to land NOW? Maybe. Maybe not.
Also, accidents aren't usually caused by one thing going bad. It's when 2 or 3 things happen at once. Meaning, a bad pilot will not automatically have an accident just because he or she isn't an experienced or qualified pilot. But a bad pilot combined with weather combined with some sort of mechanical or electrical failure is recipe for disaster. Any one of those events by itself would typically not result in a crash. So just because planes aren't falling out of the sky doesn't mean they aren't more likely to have some sort of problem.
That being said, I would have no qualms about flying Norwegian Air.