1 Install the language pack you need/want your PC to be in. (this can be done in language and region settings)
2 Open command prompt in admin mode.
3 Type lpksetup /u Into cmd prompt and you should see a window like this: https://i.imgur.com/qwltu8F.png
4 Open Regedit in admin mode, then go to: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Nls\Language
5 Find the language code of the language you installed on this webpage: https://msdn.microsoft.com/en-us/library/cc233982.aspx
6 Edit the Default and InstalLanguage values to your preferred language value, For British English the value is 0809
7 Restart your PC.
8 Open command Prompt in admin mode again and enter lpksetup /u This time your preferred language will be set as the system default.
9 Select the language you didn't want and remove it. Your PC will need to restart again and you should be done!
10 You can check if it worked by opening cmd prompt in admin mode and typing dism /online /get-intl Read through and you should see your default system language is set correctly.