Very few types change name from Japanese to English and even less completely change meaning. Fairy -> Light is pretty much less than 0% chance of happening. Fairy Breed Group is Fairy Egg Group in Japan. Since Fairy is Fairy, if it became a Type that would carry over to the English titles as Fairy as well.
What ever type we get will most likely be known in Japan and in English by the same thing or if Japanese is different, then the English will translate to the same thing. So what ever Japanese is for Light will be Light if Light appears. Same for Fairy.