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JavaScript

function isPalindrome(sentence) {
  var normalizedSentence = sentence.toLowerCase().replace(/\s/g, '').split('');
  return normalizedSentence.join('') === normalizedSentence.reverse().join('');
}

console.log(true === isPalindrome('Аргентина манит негра'));
console.log(true === isPalindrome('Аргентинаманитнегра'));
console.log(false === isPalindrome('Агрентинаманитнегра'));
console.log(false === isPalindrome('Аргентинаi манит негра'));