Yes, Apple Watch (from watchOS 9.1 with Apple Watch Series 5, Apple Watch SE, Apple Watch Ultra and later) and the Samsung Galaxy Watch support international roaming and can be used abroad. Please note that the requirements for VoLTE Roaming must be met.
Some devices cannot receive a roaming SMS so you may not receive an SMS with the price information for using the device abroad. The costs for use abroad are available in the tariff overview.