> Because that wouldn't solve the problem for clones of other sites, or clones in other languages.

Sites that are the victims of content cloning have to be very visible and valuable, so maybe a little manual curating could be relevant.

> the Stack Overflow cloners could just make other websites

Not really? The point is not to tag the clones but to tag the original; everything that is not the original and that has copied content is a clone -- its name, domain or country notwithstanding.