add contacts to an apple watch

This is one of the limitations using your iPhone to set up an Apple Watch for someone else using Family Sharing.

From the Apple Link on this topic:

Set up shared contacts and Schooltime

Now you're asked to set up the contacts that will be available on the Apple Watch. To allow this, you need to have Contacts in iCloud enabled. Go to Settings > [your name] > iCloud on your iPhone and make sure that Contacts is turned on.*

Then you can choose trusted people from your own Contacts app to share on the Apple Watch. You can manage these shared contacts later, and set up restrictions, in Screen Time on your iPhone.

You cannot however, add contacts directly on the watch, so contacts will need to be shared from your iPhone to the watch you set up in Family Sharing.

You may find the entire link on Family Sharing for Apple Watch helpful --> Set up Apple Watch for a family member - Apple Support

You'll need to contact Apple Support for help. I don't honestly know if a watch can be set up and used in a different country from where the iPhone is.

Use the Get Support button at the top of this page.