SB 9.16.33
ye madhucchandaso jyesthah
kusalam menire na tat
asapat tan munih kruddho
mleccha bhavata durjanah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When requested by their father to accept Sunahsepha as the eldest son, the elder fifty of the Madhucchandas, the sons of Visvamitra, did not agree. Therefore Visvamitra, being angry, cursed them. “May all of you bad sons become mlecchas,” he said, “being opposed to the principles of Vedic culture.”
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
In Vedic literature there are names like mleccha and yavana. The mlecchas are understood to be those who do not follow the Vedic principles. In former days, the mlecchas were fewer, and Visvamitra Muni cursed his sons to become mlecchas. But in the present age, Kali-yuga, there is no need of cursing, for people are automatically mlecchas. This is only the beginning of Kali-yuga, but at the end of Kali-yuga the entire population will consist of mlecchas because no one will follow the Vedic principles. At that time the incarnation Kalki will appear. Mleccha-nivaha-nidhane kalayasi karavalam. He will kill all the mlecchas indiscriminately with his sword.