Five years ago, Kylian Mbappé almost quit the French national team. France, a favorite in the Euro 2020 tournament, had just lost to Switzerland in the Round of 16. The French led for most of the game, only for the Swiss to score two late goals and force a penalty shootout. Mbappé ended up taking the deciding shot. The kick was strong, but the keeper reacted swiftly to bat it away. The loss was catastrophic. ESPN called it a “huge failure.” The cover of Le Parisien, a French newspaper, the next day read “Disillusionment.” On L’Equipe: “Devastated.” For Mbappé, just...