So it turns out that the Overlord language is a cipher, much like Grongi, and the Japanese captions actually transcribe it. As it happens, their names also have meaning (Demushu = Shinku = Crimson, Remue = Hisui = Jade).
These kinds folks already deciphered the dialogue so my partner from Imagination Station looked it over. Scene by scene:
Demushu vs. Baron Scene:
Demushu: {Is that the best you can do}
Demushu: {Human, more!}
Demushu: {You're boring me, human.}
Demushu: {What?!}
Demushu: {Human!}
Demushu: {Meddlesome pest...}
After the Fight:
Redue: {Hmm... so they're...}
Scene with Sagara:
Demushu: {Are you really interested in that thing?}
Redue: {It tides over my boredom.} Their language is interesting
Demushu: {Tch. Pointless.}
Sagara: {Still have too much free time?} You never change, do you?
Demushu: {You? Damn snake.}
Redue: {You're assuming their form now?}
Sagara: {I'll teach you one of their sayings:} I believe it goes “Unpreparedness is one's greatest enemy.
Gaim overhears them, before fighting with Sid:
Redue: {Isn't their language just so interesting?}
Demushu: {Fine. I can learn it as well.}
Gaim vs. Overlords:
Demushu: {Does this one have more backbone than the other guy?}
Demushu: {Come at me!}
Demushu: {I'll fight seriously... and kill you.}
Demushu: {Hmph. I expected more.}
Demushu: {What a letdown.}