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
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
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.