We're looking not just for anyone who wants to sing. We're looking for people who are in love -- madly in love -- with God and the Church. They are the ones who sing best.
Cantare amantis est. Singing is for the one who loves.
English liturgical texts can be sung to Gregorian melodies, and they often sound okay. But, most scholars of sacred music would not recommend such adaptation because the Gregorian melodies are intimately tied to the Latin language. (Besides, why use English and other vernaculars? Latin is the official sacral language of the Roman Liturgy.) Such adaptation, according to scholars, would be awkward both to the text and the melodies. There are, however, English texts set to English chant tones (often inspired by Gregorian melodies).