The Merseburger Zaubersprüche are the most famous spells, I think. Then you have this site here but it doesn't have translations nor is it in English. As far as I remember the lecture there are some more but before I can say more I have to find that book the professor recommended. And you couldn't use all spells in Fire Emblem since some of them include Norse gods or Christ and Judas. In some cases you could probably cut those parts out.