ваш код работает правильно
func isCorrect(word:String)->Bool{
let checker = UITextChecker()
let range = NSRange(location: 0, length: word.utf16.count)
let mispelledRange = checker.rangeOfMisspelledWord(in: word, range: range, startingAt: 0, wrap: false, language: "en")
return mispelledRange.location == NSNotFound
}
print(isCorrect(word: "apple"))
print(isCorrect(word: "ppale"))