Weren't they written in greek?
Edit: Incidentally, a simple Google would've shown it was written in Greek: http://en.wikipedia.org/wiki/Meditations
Never trust people on the internet, even yourself.
Graecia capta ferum victorem cepit et artes intulit agresti Latio. - Ep. II.i.156
"Greece, once conquered, herself conquered the artless victor, and planted the banner of civilization in the farmlands of Rome."
I have translated freely and taken considerable license.
By Aurelius' time, familiarity with Greek for the ruling classes was a given; as a matter of preference, it is a more supple language and probably more comfortable to use for putting abstract reasoning to words.