Mleccha means not following the Vedic principles. Those who are following the Vedic principles, they are classified guna-karma-vibhagasah [Bg. 4.13], in four divisions. And beyond these four divisions, who are less than sudras, they are called mlecchas and yavanas and candalas. Their behavior is so abominable that they cannot be accepted as civilized man. Civilized man, not by so-called material advancement. Civilized man is this four division of society - brahmanas, ksatriyas, vaisyas, and sudras. That is civilized society. Where there is no such division, everyone is a sudra, that is not a civilized form of society.
Lecture on Srimad-Bhagavatam 6.3.12-15, Gorakhpur, February 9, 1971