I'm not sure where you got the translation from, but it should mean:
"no longer do I say that you are slaves" or more hyperliterally, "no
longer do I [say =] call you slaves." Depending on your theory of
grammar, you can either think of it as LEGW governing a double direct
object or that the direct object of LEGW is an infinitive clause with
an understood EINAI "to be." The subject and complements of EINAI are
in the accusative.
The proposed translation seems to understand hUMAS as some kind of a
dative form (hUMIN), with DOULOUS being in apposition to it. However,
for that to work, both must be in the dative, but they're not: they are
accusative. A full parsing is:
OUKETI - adv. modifies LEGW "no longer"
LEGW - vb. 1s pres act ind of LEGW "I say, speak, etc."
hUMAS - pron. acc 2p, subj of [EINAI] "you"
[EINAI] - vb. pres act inf of EIMI, obj of LEGW "[to be]"
DOULOUS - n. acc p of DOULOS, cmplmt of [EINAI] "slaves"
-- Stephen C. Carlson, George Mason University School of Law, Patent Track, 4LE firstname.lastname@example.org : Poetry speaks of aspirations, and songs http://osf1.gmu.edu/~scarlso1/ : chant the words. -- Shujing 2.35