When I go to that section as a Domain User the 'Enable spelling correction' is already ticked and 'English (United Kingdom)' is selected as default. If I click any other language then the button above 'Close' does appear but it is also blank and the 'Enable spelling correction' is unticked.
Logging in as Domain Admin account turns the blank button to 'Set As Default'
Nice job Microsoft
Regards,
ICT Tech (@manwithnoname)