This worked for me as suddenly – after upgrading to Sierra – the App Store was all Dutch where previously it was English:
After I fought with the issue for a few hours, I found the solution.
- Open Mac App Store and log out of the account.
- Restart your Mac.
- Log in to the account again.
Thanks [WayBack] Blaszard for the above answer at [WayBack] macos – How to change the language used on Mac App Store? – Ask Different
Note that this syncs the language in the App Store with the one you’ve set on the OS X level; for changing that, see [WayBack] macOS Sierra: Change the language your Mac uses
–jeroen