SAINT-DENIS, France — Filled with the savory scent of kebab shops and the lilting sounds of foreign-accented French, this sprawling suburb north of Paris is the hometown of a not-so-proud son: Jordan Bardella. In scathing critiques, the 28-year-old seeking to be this country’s youngest prime minister cites the multiculturalism here as an example of everything that is wrong with today’s France.