DMCA take downs are only effective because Google is a US company. If I have a company in e.g. Slovenia, DMCA is ineffective, and they would have to go through Slovenian courts to take down the website (unless they just seize the domain).

Yes and no. Some countries (Canada being one) have treaties with the US that let court judgements in one country be enforced in the other. So even if you have no assets in the US or income from the US, a US court decision could still affect you if you live in such a country.

Of course, most internet companies do do some business in the US.

Good point. Obviously Google does considerably more business in the US than Canada, but it's still bizarre for any one country's courts to make decisions that directly impact the entire world.

Maybe it's time for an international treaty on Internet regulation to spell out exactly what powers any given nation has. (Although such a thing would probably have the potential to go very wrong.)

DMCA takedowns are not court orders. You're not actually compelled to comply with them at all, but you lose access to the safe harbor provision if you don't.