I have found a lot of information that this character is a tabaxi and speaks tabaxi language.
Ras Nsi before he became yuan-ti
Who he is exactly? Was he tabaxi or human or something else?
The book makes this all very confusing, but Ras Nsi is indeed a Tabaxi. Except... he's not a cat person. In ancient Chult, the people of Chult lived in various tribes, and the largest of these tribes was the Tabaxi tribe.
It's a bit uncertain in-universe if the tribe was named after the cat-like humanoids, or if the misunderstanding came from an explorer who asked about a cat-like humanoid from the Tabaxi tribe, was told they're a Tabaxi, and assumed that 'tabaxi' was the race of the creature instead of what tribe it belonged to.
To make matters even more confusing, the language he speaks (Tabaxi) is not the same language as the one the cat-people speak. In fact, if you look closely, the tabaxi species doesn't even speak Tabaxi, because Tabaxi is a human dialect from Chult. The language the furry friends spoke in earlier editions when they still had their own language by default is a dialect of Payit, completely unrelated to human Tabaxi.
This difference is properly explained in Dragon+ #11 in Matt Sernet's Lore You Should Know article, "Tabaxi vs. tabaxi".