Who first described Pentecost as a reversal of Babel? In Spiritual Maturity, J. Oswald Sanders describes Pentecost as a reversal of Babel. An online search shows this to be a link made by others, and in a related thesis on the topic, Paul J. Pastor goes as far as to claim:


  Pentecost as a reversal of Babel has been widely seen by exegetes since the early days of the Church.


Who are some of these early exegetes?
Two fourth-century fathers clearly make this connection. First, Cyril of Jerusalem:


  The multitude of the hearers was confounded;—it was a second confusion, in the room of that first evil one at Babylon.  For in that confusion of tongues there was division of purpose, because their thought was at enmity with God; but here minds were restored and united, because the object of interest was godly. (Lecture 17.17)


Gregory of Nazianzus, during his discussion of Pentecost, contrasts Babel:


  But as the old Confusion of tongues was laudable, when men who were of one language in wickedness and impiety, even as some now venture to be, were building the Tower; for by the confusion of their language the unity of their intention was broken up, and their undertaking destroyed; so much more worthy of praise is the present miraculous one [i.e., the tongues at Pentecost]. For being poured from One Spirit upon many men, it brings them again into harmony. (Oration 41)


Augustine, shortly thereafter, clearly makes the connection in his exposition of Psalm 55:9:


  Recollect that tower of proud men made after the deluge [...] they builded up a lofty tower, and the Lord divided the tongues of them. [...] Through proud men, divided were the tongues; through humble Apostles, united were the tongues. Spirit of pride dispersed tongues, Spirit Holy united tongues. For when the Holy Spirit came upon the disciples, with the tongues of all men they spake, by all men they were understood: tongues dispersed, into one were united. (Psalm LV.10)


A number of later fathers continued this understanding.  In the sixth century, we have Pseudo-Fulgentius (quoted in Henri de Lubac, Catholicism) and, quoted here, Arator:


  Then there was a confusion of tongues, though they were of the same race; now there is one tongue though the peoples are many. (Quoted in Hiller, Arator on the Acts)


And in the early 8th century, Bede refers to the Church speaking various languages at Pentecost:


  How the arrogance of Babylon scattered the unity of languages, [and] the humbleness of the Church gathers it back. (Commentary on Acts)