Thanks go to Gatsu the gamebreaker, who noted this. I did a research and realized that there isn't any other Poké-Body in Pokémon history, except for this one, that gives you the possibility to choose whether using the Body or not. And that's why it's a Poké-Power "thing". There is no mistranslation because Flygon was released in the USA before Japan (and even the japanese version has the "you may" thing). The card should be errata'd, taking out the "you may" or changing Psychic Protector into a Poké-Power, shouldn't it?