Apple Watch SE
I heard that the Apple Watch is not compatible to other counties phone signal systems. Should I wait until it will be compatible? How can I use my watch when I am not in the USA?
As fieryjack says when away from your iPhone carrier's service area, with your iPhone nearby, you can still do everything you did before, only now using the iPhone as a relay.
The Apple Watch does not support roaming. As it is, to save on size and power, there are 2 different radio chips used. One for Canada, U.S., Mexico and Puerto Rico, and one for the rest of the world.
When cellular chips become sufficiently power and size efficient and can also service all the different radio frequencies used around the world, then maybe the Apple Watch will be able to support roaming.
So do not wait for a mythical future capability. Just buy the Apple Watch you want today, and enjoy it. And when you travel, just keep your iPhone nearby (where nearby can be in the same hotel room, or you pocket, backpack, briefcase, etc...).
