Buy watch for German cellular network in the US

If you check this list of wireless support for the Apple Watch, Apple Watch - Carriers - Apple you will see the model numbers for the US and Germany differ. You would need to purchase the device in Germany to use there. If you purchase and use a US device, you probably will not be able to use it in Germany, since it wouldn't support the correct bands. If your iPhone is unlocked, you would be able to use the Apple Watch with the iPhone like you would any other Apple Watch without cellular, but it seems the model numbers for the German devices are different than the US ones.

Hi

Apple does not sell any European models of Apple Watch Series 3 (GPS + Cellular) within the US.

Apple Watch Series 3 (GPS + Cellular) is not locked to any specific carrier and has a built-in eSIM that copies information from the iPhone's SIM card.

The built-in cellular features require a carrier plan for your iPhone in your country of use that specifically includes support for Apple Watch Series 3 (GPS + Cellular), including the carrier activating the device on the plan.

In order to be able to use the built-in cellular feature in another country, you would need to buy a model that supports the relevant LTE and UMTS bands for your intended country of use and your supported carrier there.

There are three different regional versions of Apple Watch Series 3 (GPS + Cellular), each supporting different LTE and UMTS bands, as detailed here - Apple lists different models for the US and Germany:

Apple Watch - Carriers - Apple

If in doubt, I suggest that you contact your intended carrier in Germany to ask whether any given model (such as a US model) can be used on their network with a plan that they may offer for iPhone that supports Apple Watch Series 3 (GPS + Cellular).

Apple Watch Series 3 (GPS + Cellular) can be used without activating the built-in cellular feature, in which case it functions like a GPS-only model, in conjunction with an iPhone 6 or newer model of iPhone (running the latest version of iOS) and a service plan from any carrier.

The built-in cellular features cannot be used when roaming.

The warranty terms for an Apple Watch purchased in the US are detailed here:

Apple One (1) Year Limited Warranty