The only swear word Tamaki used in the anime (at least in the dub) is “bastard” which makes sense because he’s been privately tutored in Japanese, his tutor wouldn’t teach him swears. So the only one he knows is the one people kept calling him. ie “bastard”
Fun Fact: 5 year old Kyoya tripped over himself to show off how much he knew about oranges. In the manga, he literally made an enemy (Kuze) because he could not resit being an attention seeking know-it-all in front of all his class. This benefited him in no way. In fact it obviously made him worse off.
When Kyoya was a kid he was young and earnest and desperate to prove how smart he was and be validated (I wonder where that comes from, almost as if, he felt insecure, like he was comparing himself to older siblings or something…hmmm)
Kyoya wasn’t born the Shadow King/Demon Lord. Somewhere along the way he broke the five year old who just wanted people to pay attention to him and had no tact. He broke the five year old who wasn’t good enough. He broken himself down until none of that weakness was left, until he could live and breath the shadows. And then he built himself a new persona, one that was entirely fake, and strong.