Melkor sabia que os homens acabariam surgindo
Embora ele não fosse tão sábio quanto os outros Valar, preferindo gastar seu tempo desafiando Eru em vez de ouvir a música, ele ainda sabia que os humanos eventualmente viriam a ser.In those days, moreover, though the Valar knew indeed of the coming of Men that were to be, the Elves as yet naught of it; for Manwë had not revealed it to them. But Melkor spoke to them in secret of Mortal Men, seeing how the silence of the Valar might be twisted to evil. Little he knew yet concerning Men, for engrossed with his own thought in the Music he had paid small heed to the Third Theme of Ilúvatar; but now the whisper went among the Elves that Manwë held them captive, so that Men might come and supplant them in the kingdoms of Middle-earth, for the Valar saw that they might more easily sway this short-lived and weaker race, defrauding the Elves of the inheritance of Ilúvatar.
—The Silmarillion
Os Valar certamente acreditavam que Morgoth poderia procurar humanos:
These things the Valar did, recalling in their twilight the darkness of the lands of Arda; and they resolved now to illumine Middle-earth and with light to hinder the deeds of Melkor. For they remembered the Avari that remained by the waters of their awakening, and they did not utterly forsake the Noldor in exile; and Manwë knew also that the hour of the coming of Men was drawn nigh. And it is said indeed that, even as the Valar made war upon Melkor for the sake of the Quendi, so now for that time they forbore for the sake of the Hildor, the Aftercomers, the younger Children of Ilúvatar.
—The Silmarillion
E como o Melkor pode se comunicar com os homens?
Melkor poderia tocar as mentes dos outros
Como indicado por @Ber, Melkor, como todo Vala, tinha uma grande capacidade de comunicar mente a mente e, crucialmente, saber o que os outros estavam pensando.
For he would come by stealth to a mind open and unwary, hoping to learn some part of its thought before it closed, and still more to implant in it his own thought, to deceive it and win it to his friendship.
—Osanwë-kenta
Através disso, ele poderia aprender a língua dos homens, elfos, ou qualquer outra coisa, desde que eles não fossem cautelosos com ele.
Melkor pode ter aprendido a língua deles, ou ensinado a eles o seu próprio
Ele também era conhecido pelos Valar por ter uma grande facilidade com a linguagem, tal era seu desejo de usá-lo para subverter a resistência de outros para dirigir a aplicação de sua vontade."From the first he was greatly interested in 'language,' that talent which the Eruhini would have by nature; but we did not at once perceive the malice in this interest, for many of us shared it, and Aule above all. But in time we discovered that he had made a language for those who served him, and he has learned our tongue with ease. He has great skill in this matter. Beyond doubt he will master all tongues, even the fair speech of the Eldar. Therefore, if ever you should speak with him, beware!"
—Osanwë-kenta
Isso sugere que, qualquer que seja a linguagem que os primeiros seres humanos possuíssem, Melkor poderia tê-lo adquirido, especialmente considerando que, a princípio, eles não eram cautelosos.
De fato, é possível que ele tenha ensinado a alguns humanos sua própria língua, como indicado no texto anterior, o que facilitaria a comunicação e tornaria mais fácil moldá-los para seus propósitos.