The text on Mephisto's says something along the lines of "fili dei miserete xxxx" I can't read the last word, it's too small...
Is that latin? What does it mean?
It's Latin or more probably manga Latin. How can you even read it? It's so small.
"filii dei" means "sons of god" ("filius" would be "son")
"miserere" is pleading for mercy/pity ("have mercy" vocative case) I'm not sure there is a form "miserete".
These are common phrases from most popular Latin prayers (before Second Vatican Council every Catholic knew a couple of them by heart), so Katou could've not know a word in Latin and still come up with these.