Apple Watch Series 9 only GPS

As Alex says, it will function anywhere your iPhone functions, as it does not depend on any cellular carrier, just the paired iPhone.

However, the warranty is country specific, so you may need to return to the Czech Republic (or another EU country) to get warranty service, or pay out-of-pocket.

Some countries put restrictions on Apple Watch features. You should verify that all the features you want are enabled on a Czech Republic sold Apple Watch

watchOS - Feature Availability - Apple

It doesn’t matter where you purchase the Apple Watch (GPS + Wi-Fi) because it will work the same no matter where you are and according to the standards and terms of the location where it is used.

Axel F.