The meat-eaters are generally called mlecchas. But all meat-eaters are not mlecchas. Those who accept meat in terms of scriptural injunctions are not mlecchas, but those who accept meat without restriction are called mlecchas. Beef is forbidden in the scriptures, and the bulls and cows are offered special protection by followers of the Vedas. But in this age of Kali, people will exploit the body of the bull and the cow as they like, and thus they will invite sufferings of various types.
(Srimad Bhagavatam----1:16:20----purport).

There are gradations of human beings in terms of proportionate faith in the Supreme Personality of Godhead. The first-class faithful men are the Vaiṣṇavas and the brāhmaṇas, then the kṣatriyas, then the vaiśyas, then the śūdras, then the mlecchas, the yavanas and at last the caṇḍālas. The degradation of the human instinct begins from the mlecchas, and the caṇḍāla state of life is the last word in human degradation. All the above terms mentioned in the Vedic literatures are never meant for any particular community or birth. They are different qualifications of human beings in general. There is no question of birthright or community.
(Srimad Bhagavatam----1:16:20----purport).

After sufficient rains fall, there is sufficient production of grains for men and all animals, and thus there is energy in the living being for progressive activity. Themlecchas, however, make plans to install slaughterhouses for killing bulls and cows along with other animals, thinking that they will prosper by increasing the number of factories and live on animal food without caring for performance of sacrifices and production of grains. But they must know that even for the animals they must produce grass and vegetables, otherwise the animals cannot live. And to produce grass for the animals, they require sufficient rains.
(Srimad Bhagavatam----1:16:20----purport).

You are given the chance of becoming brāhmaṇa. You should not become again śūdras or mlecchas or caṇḍālas. Be careful. This is brāhmaṇa business. If we again come back to caṇḍālas or śūd... Caṇḍāla means less than the śūdras. Even śūdras, they eat meat under control. But caṇḍālas, they eat meat without any control.
( Lecture on Srimad Bhagavatam--6:1:61).

The modern Sind province formerly extended on the other side of the Arabian Sea, and all the inhabitants of that province were known as the Ābhīras. They were under the domination of Mahārāja Yudhiṣṭhira, and according to the statements of Mārkaṇḍeya the mlecchas of this part of the world would also rule over Bhārata. Later on this proved to be true, as in the case of the Pulindas. On behalf of the Pulindas, Alexander the Great conquered India, and on behalf of the Ābhīras, Muhammad Ghori conquered India. These Ābhīras were also formerly kṣatriyas within the brahminical culture, but they gave up the connection.
(Srimad Bhagavatam----2:4:18----purport).

Therefore the Turks are Yavanas due to being descendants of Mahārāja Yavana. The Yavanas were therefore kṣatriyas, and later on, by giving up the brahminical culture, they became mleccha-yavanas. Descriptions of the Yavanas are in the Mahābhārata (Ādi-parva 85.34). Another prince called Turvasu was also known as Yavana, and his country was conquered by Sahadeva, one of the Pāṇḍavas. The western Yavana joined with Duryodhana in the Battle of Kurukṣetra under the pressure of Karṇa. It is also foretold that these Yavanas also would conquer India, and it proved to be true.
(Srimad Bhagavatam----2:4:18----purport).

There is no bar to propagating the Kṛṣṇa consciousness movement even among people who are born in caṇḍāla, mleccha or yavana families. Even in India, this point has been enunciated by Śrīla Sanātana Gosvāmī in his book Hari-bhakti-vilāsa, which is smṛti and is the authorized Vedic guide for Vaiṣṇavas in their daily behavior. Sanātana Gosvāmī says that as bell metal can turn to gold when mixed with mercury in a chemical process, so, by the bona fide dīkṣā, or initiation method, anyone can become a Vaiṣṇava. One should take initiation from a bona fide spiritual master coming in the disciplic succession, who is authorized by his predecessor spiritual master.
(Srimad Bhagavatam----4:8:54----purport).

In the Twelfth Canto of Śrīmad-Bhāgavatam (12.1.40) there is a description of these thieves and rogues in government service. As stated, prajās te bhakṣayiṣyantimlecchā rājanya-rūpiṇaḥ: "These proud mlecchas (persons who are less than śūdras). representing themselves as kings, will tyrannize their subjects, and their subjects, on the other hand, will cultivate the most vicious practices. Thus practicing evil habits and behaving foolishly, the subjects will be like their rulers." The idea is that in the democratic days of Kali-yuga, the general population will fall down to the standard of śūdras.
(Srimad Bhagavatam----4:14:17----purport).

The words yavanānām ṛṣabham refer to the King of the Yavanas. The Sanskrit words yavana and mleccha apply to those who do not follow the Vedic principles. According to the Vedic principles, one should rise early in the morning, take bath, chant Hare Kṛṣṇa, offer maṅgala-ārati to the Deities, study Vedic literature, take prasāda and engage in dressing and decorating the Deities. One must also collect money for the temple expenditures, or if one is a householder he must go to work in accordance with the prescribed duties of a brāhmaṇa, kṣatriya, vaiśya or śūdra. In this way one should live a life of spiritual understanding, and this is the Vedic way of civilization.
(Srimad Bhagavatam----4:27:24----purport).

One who does not follow all these rules and regulations is called a yavana or mleccha. One should not mistakenly think that these words refer to certain classes of men in other countries. There is no question of limitation according to nationalism. Whether one lives in India or outside of India, he is called a yavana or mleccha if he does not follow the Vedic principles. One who does not actually follow the hygienic principles prescribed in the Vedic rules and regulations will be subjected to many contagious diseases.
(Srimad Bhagavatam----4:27:24----purport).

Those who do not follow the Vedic principles are unrestricted as far as sex life is concerned. Consequently they sometimes do not hesitate to have sex with their sisters. In this age of Kali there are many instances of such incest. Although Yavana-rāja accepted the request of Nārada Muni to show respect to him, he was nonetheless thinking of illicit sex. This was due to his being the King of the yavanas and mlecchas.
(Srimad Bhagavatam----4:27:30----purport).

Even if one is a brāhmaṇa and is qualified with all the brahminical qualifications, he is considered degraded if he is averse to worshiping the Supreme Personality of Godhead. But if a person is attached to the service of the Lord, he becomes glorified even if he is born in a caṇḍāla family. Indeed, such a caṇḍāla can deliver not only himself but all his family predecessors. Without devotional service, even a proud brāhmaṇa cannot deliver himself, and what to speak of his family. In many instances in the śāstras it is seen that even a brāhmaṇa has become a kṣatriya, vaiśya, śūdra, mleccha or non-brāhmaṇa. And there are many instances of one's being born a kṣatriya or vaiśya or even lower and, in the eighteenth year, attaining elevation to the brahminical platform by the process of initiation.
(Srimad Bhagavatam----4:31:10----purport).

Unfortunately, because of the influence of Kali-yuga, everything is now being lost. The inhabitants of Bhārata-varṣa are gradually becoming degraded mlecchas and yavanas. How then will they teach others? Therefore, this Kṛṣṇa consciousness movement has been started not only for the inhabitants of Bhārata-varṣa but for all the people of the world, as announced by Śrī Caitanya Mahāprabhu. There is still time, and if the inhabitants of Bhārata-varṣa take this movement of Kṛṣṇa consciousness seriously, the entire world will be saved from gliding down to a hellish condition.
(Srimad Bhagavatam----5:19:10----purport).

These so-called brāhmaṇas give sanction to slaughterhouses for the sake of a fat salary, and they do not protest these abominable activities. By deprecating the principles of Vedic civilization and supporting cow-killing, they are immediately degraded to the platform of mlecchas and yavanas. A mleccha is a meat-eater, and a yavana is one who has deviated from Vedic culture. Unfortunately, such mlecchas and yavanas are in executive power. How, then, can there be peace and prosperity in the state? The king or the president must be the representative of the Supreme Personality of Godhead.
(Sri Caitanya Caritamrta----2:1:197----purport).

The material position of a devotee doesn’t matter because devotional service is not dependent on material considerations. In his earlier life, Śrīla Rūpa Gosvāmī was a government officer and a gṛhastha. He was not even a brahmacārī or sannyāsī. He associated with mlecchas and yavanas, but because he was always eager to serve, he was a qualified recipient for the Lord's mercy. A sincere devotee can therefore be empowered by the Lord regardless of his situation. In the preceding verse from the Bhakti-rasāmṛta-sindhu, Śrīla Rūpa Gosvāmī has described how he was personally empowered by the Lord.
(Sri Caitanya Caritamrta----2:19:135----purport).

"There are many different kinds of devotees, but even a Vaiṣṇava coming from a family of mlecchas or yavanas is understood to be a learned scholar, complete in knowledge, if he knows the Vaiṣṇava philosophy. He should therefore be given charity, for such a Vaiṣṇava is as worshipable as the Supreme Personality of Godhead."
(Sri Caitanya Caritamrta----3:3:222----purport).