Because no one here knows the most popular men’s Christian name in 🇷🇴 Romania or Hungary 🇭🇺
This is true. Sergei it is then.

Actually, the name is Latin in origin, and hence Roman.
And as Romanian is one of the Latin languages, it's arguably also a Romanian name.
The Russians borrowed it from the French name, Serge, (also a Latin language), so it's easy to argue it's not a Russian name, just a popular man's name in Russia that's an adaptation of the French original.