I found a chrome extension called 'Dominion_kor'.
When I added the extension and started the game, The names and descriptions of the cards changed to Korean.
Is this extension illegal?
Strictly speaking, it mit might be against the terms of service. But if it "just" changes card texts to another language, I don't think it will be a problem, there are other tolerated extensions for the game.
The big question is always:
Do you, and the game's owner, trust the extension not to do nefarious things?
And I guess the potential damage to you (if the extension phishes your online banking passwords) is bigger than the potential damage to the game (you can't cheat by pretending to play a King's Court if you don't have one in hand, because the server knows what cards you have in hand).
Is this extension "illegal"?
If I use this extension in Dominion Online, Can I get banned because of the extension?
I tried to answer the question as well as I could, as a non-lawyer. What ultimately happens when you are banned because of this, and you chellange that in a court of law, is something I have absolutely no hope to predict or even guess.