Swift 3 solution:
let languages = ["bs", "zh-Hant", "en", "fi", "ko", "lv", "ms", "pl", "pt-BR", "ru", "sr-Latn", "sk", "es", "tr"]
UserDefaults.standard.set([languages[0]], forKey: "AppleLanguages")
Gave some examples of language codes that can be used. Hope this helps