Changed macbook region, but system language has not changed

If you add say English & that doesn't work...

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode? Could take 10 minutes.

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

To find out if it's system wide or user specific, try this...

Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

Does it work in the new account?

Should work, I did tell you I changed mine from English to Turkish and back.

Are you sure you are doing it correctly.

Have you removed Turkish from the preferred language section.

Which macOS are you on, if you are on macOS High Sierra you can download the installer

and reinstall the OS.

This does not mean you need to wipe the mac and then do a clean install, just install over what you have.

When the installation finishes and the mac goes through the Set Up procedure here you can tell the mac

about the region and language required and this should clear the error.

Even though you are going to reinstall over what you have, make a full backup of your mac.

Open your Safari browser and click on the link below,

How to get old versions of macOS – Apple Support (UK) Go to the Download section and click on macOS High Sierra.

This will redirect you to the App Store High Sierra downloaded page.

It should say Get or Download. This will download the Install macOS High Sierra.app

to your Applications folder it should be 5.23GBs in size.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS High Sierra.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

I have just switched to Turkish and then back again to English everything worked as expected.

System Preferences> Language and Region

Select Region: United Kingdom

In the left hand panel select English as your primary language

using the + button.

Click on the Show All Button, the square button in the Toolbar at the top of System Preferences

You will be asked if you want to restart press Restart Now,

the changes will be applied on the restart.

The samples below show the change from English to Turkish, just swap it over for your situation.