A USA purchased Apple Watch with cellular will behave like the non-cellular model in Europe and anywhere else which does not use the North American regional LTE frequencies. It will NEVER work on cellular in Europe, even if our Czech carrier announces support for Apple Watch, the radio frequency bands are not compatible. Apple Watch - Carriers - Apple
My advice is not to waste money on the USA cellular version unless you intend to relocate to USA. Buy the non-cellular model, or the European cellular version if you think Czech carriers may provide support within the life of your new watch.