Since the Apple Watch shares the iPhone's phone number, the iPhone carrier MUST support the Apple Watch.
If you obtained a cellular model from another country, it would still act the same as a GPS only model, which does everything the cellular model does AS LONG AS your iPhone is nearby.
The only advantage to getting a cellular model is that you have the option of different cases, and paying more money. It will not give you cellular service until a carrier in your country supports the Apple Watch, and then you would need to switch your iPhone to that carrier.
It would be best if you talked to the carriers to at lest them know your interest. Maybe if enough customers do that, they will consider providing support.