The IEEE has approved a single “global“ frequency for zigbee, 2.4 GHz, so we usually say that there is only one zigbee frequency worldwide. And that’s true for the global standard.
However, that’s the same frequency that Wi-Fi operates at, and a few regions are so congested in that band that the zigbee alliance has approved some different regional frequencies.
There are three of these for the EU, and they are becoming increasingly popular, particularly since the UK has a new requirement coming that all appliances be able to communicate with smart meters. This will be done using zigbee on the 868,3 frequency.
A good example are Bosch devices. Although their global frequency zigbee devices like the motion sensors sold in the US generally work well with the smartthings hub, the light switches sold in Germany are zigbee 868,3 and cannot communicate with a smartthings hub.
(Note That zwave In the EU is on a slightly different frequency, 868,4 )
I Haven’t heard of any plans to add a regional zigbee frequency radio to Any smartthings hubs in the EU, although there may be some market pressure to do so in the UK once the new laws go into effect.
Also my thanks to @jelockwood Who first brought the UK regional zigbee frequency issue to my attention last year during discussion of smart meters.