The following statement by Yudhistrira clarifies that foreigners are aware of (benefits of) fasting during Dwapara Yuga
"Yudhishthira said, 'The disposition is seen, O grandsire, in all the orders of men, including the very Mlechchhas, of observing fasts. The reason, however, of this is not known to us. It has been heard by us that only Brahmanas and Kshatriyas should observe the vow of fasts. How, O grandsire, are the other orders to be taken as earning any merit by the observance of fasts? How have vows and fasts come to be observed by persons of all orders, O king? What is that end to which one devoted to the observance of fasts attains? It has been said that fasts are highly meritorious and that fasts are a great refuge. O prince of men, what is the fruit that is earned in this world by the man that observe fasts? By what means is one cleansed of one's sins? By what means doth one acquire righteousness? By what means, O best of the Bharatas, doth one succeed in acquiring heaven and merit? After having observed a fast, what should one give away, O king? O, tell me, what those duties are by which one may succeed in obtaining such objects as lead to happiness?'
Do any other statements from any scripture comment on this? I mean, do any other statement from any scripture also mention that foreigners were aware of fasting, which is one of the integral ritual of Sanathan dharma, in previous yugas.
Note: I want answer only from scriptures, as I told, not from the opinions or books by historians, archeologists, modern experts, etc., (which this site is not intended for).