Michaela DePrince was appearing here in the third act of Balanchine’s “Jewels” with the Dutch National Ballet on a recent evening. There were about a dozen ballerinas in bejeweled white tutus arrayed across the stage, and there she was all the way stage right in the front line. This 20-year-old American ballerina, who joined the Dutch National Ballet’s main company in August, is in some respects the equivalent of a chorus girl in a Broadway musical. But night after night, people show up just to see her dance. Ms. DePrince, who was an orphan in war-torn Sierra Leone and became a profe