In System Preferences : Keyboard : Text panel, set Spelling to your explicit language, not Automatic by Language. You might consider checking Correct spelling automatically in that panel too.
In Pages itself, visit Edit : Spelling and Grammar : Show Spelling and Grammar panel, and change it to your explicit language, rather than Automatic by Language. And finally, in Pages Preferences : Auto-Correction panel, verify that Correct spelling automatically is also selected.
Do you have Edit menu : Spelling and Grammar : √ Check Spelling While Typing?
With all of the above settings enabled, Pages 10 on macOS Mojave 10.14.6 still misses the spelling error in eduction, with or without leading capital. This is likely a word you will have to right-click on and add to the dictionary, though a shameful issue.