Niska speaks Czech to one of his flunkies
Dej jim za to plátek masa
This translates to "Give them a slice of meat"
This is backed up by the shooting script
NISKA: No, no, no. I insist.
(to Torturer, in Czech) They have enough for a slice.
(to Zoe, in English) I wouldn't want the talk to be that Adelai Niska is a cheat.
and official Firefly RPG Corebook
ADELAI NISKA
Mr. Adelai Niska is a wealthy crime boss with a passel of
Scary-Looking Henchmen, a torturer, and a fortified
skyplex to call his own. Unlike Badger, Niska is overflowing with
charm and is accustomed to playing the part of a fine, elderly
gentleman. One might even say he’s downright philosophical.
The Georgia System’s notorious gangster is fluent in both
Chinese and Czech, at the helm of a close-knit family, and
proudly displays a few Earth-That-Was artifacts. His polite,
grandfatherly demeanor fools a lot of folk. Underneath it all,
Niska is a criminal mastermind obsessed with protecting his
fierce reputation.
In the Serenity RPG Big Damn Heroes Supplement, his accent is referred to as "Slavic"
Niska’s voice is soft and he speaks with a Slavic accent.
It seems likely given his Earth-That-Was ties and closely knit family links, that Niska's people originally came from the Eastern European region, hence the preservation of a strong and distinctive Slavic accent.