South Korea has been slow to acknowledge the official titles of the North Korean leader Kim Jong-un since he took office exactly a year ago, sources said Thursday, reflecting the strained relations between the two Koreas.Kim, known to be in his late 20s or early 30s, inherited the country after the sudden death of his father Kim Jong-il in late 2011, and was named first secretary of the ruling Workers\' Party of Korea (WPK) the year after.