Complex sentence: By 1997 "Night" was selling 300,000 copies a year in the United States; by 2011 it had sold six million copies in that country, and was available in 30 languages.
Split into simpler sentences: By 1997 "Night" was selling 300,000 copies a year in the United States. By 2011 it had sold six million copies in that country, and was available in 30 languages.