It depends on the app. Roku has various regions that correspond to where it’s officially sold and supported — US, Canada, UK, Ireland, Mexico, France, etc — plus a “Rest of the world” region. It’s up to the app owner determine where the app will appear in the Channel Store and how well it operates there.
Basil https://www.basilsblog.com/ Roku Ultra (4660) Apple TV (5th gen), TiVo
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To add to Basil's answer, all Roku players are region specific in some manner. While you can buy one in one region and use it elsewhere, you have no guarantee that it will work correctly in other places. In addition, don't forget that different parts of the world have different AC power configuration and different TV display standards. A unit bought in North America might not work correctly on a European or Asian TV.
Dan Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.