Apple Watch GPS+Cellular and International Travel with T-Mobile DIGITS

According to this support document for Apple Watch carriers, Apple Watch - Carriers - Apple, the Watch cannot roam outside of the home carrier's network. However, you could still use the watch with the paired phone, like you can with all of the other versions of the watch.

Apple Watch doesn’t roam on cellular. As soon as you leave your home network it will fall back to working like the non cellular watch versions and all communication is through the paired iPhone. This suggests that extra data for the watch is likely to be insignificant, because most of the content on the watch is obtained by the iPhone forwarding data it has received into the corresponding apps in the phone. It should be only the things you initiate on the watch (like using Siri or maps) which generate extra traffic, and even this is debatable because if you didn’t have the watch the same activity would probably be launched direct from the phone.