MacOS Language and Country Settings
Localization on OS X and the App Store you are using are two different things.


Localization is done on User Account level, meaning you can define the language, the date format etc. individually for each user on your Mac
The App Store is tied to the Apple ID used, so the Store you use is the one the Apple ID is connected with. You can't switch stores within one ID but it's possible (even if it's a bit cumbersome in practice) to have several IDs for different stores.