Well, Dunkirk is written, directed, and acted by British people. So its more accurate to say the British movie destroyed the French one.
LOL at the idea that Dunkirk beat Valerian because its director is more well known. Yeah, the millions of movie goers main deciding factor in choosing a movie is not the director. The world barely even thinks of Valerian as a French movie. Just another shitty Hollywood one. So the idea that Dunkirk did well only because of American loyalties to American movies is just flat out preposterous.
Valerian bombed because its a bad movie, with bad word of mouth and a marketing campaign that didn't do its job. Problem is Luc Besson just doesn't seem to have a great handle on the English language, and this shines through in the atrocious dialogue and even worse line delivery in many of his movies.