Yes a Sorbian folk tale, the Snake King

Well folk legends, I even encountered a Hungarian version of it

My assumption is it is an older form of Weles

Veles, the slavic god of forests, poetry and the underworld. Snake is indeed one of his symbols. The other one being a bear btw

Sounds like whimsical chthonic entity behavior, maybe some centuries of boring christianity were too much for that Naga lord and decided to play games

Why not? The faeric beings are usually described and whimsical but also cruel and well, inhumane since they're not human

That's very similar to what in Estonian culture is known as the Snake King (ussikuningas). I've seen it represented with three heads at times. I'm not sure about it teaching the language of animals though, I'll have to look it up.

Apparently the Mordvins Central Russia have a similar entity (Inekui, 'Great snake') usually depicted with 7 or 12 heads, and it can also fly. By eating its meat one can understand the language of animals. It is a chthonic entity in control of malicious spirits, and in some stories it is won in battle by a man born of the marriage of a bear and a human woman. The half-bear is the ancestor of certain clans among Uralic peoples, such as the Skolt Saami.
https://www.folklore.ee/rl/folkte/sugri/mordva/myto/ussikuningas.htm

yooooo

Indian deity lying on a Naga