In Luceti, all characters are built with their own universal translator; namely, the wings. Speech, text, anything that involves language of any kind is automatically filtered through the changed physiology of the characters and translated into words. As for whether the character is naturally speaking/writing Luceti's native language or their own and simply perceives everything else as the same... use your imagination.
Sometimes bits and pieces of languages will filter through, particular if it's a well-known phrase the character uses, such as coup d'etat is sometimes used in English society. Just words and little phrases though, never full sentences. IC examples would be if Alphonse prefers to call Edward "niisan" instead of "brother", or someone with an accent might speak oddly to others.