Pages (Mac) not recognising US English spellings

Hi. I'm creating an article in Pages that needs to use US English spellings. Yet, words like "decentralized" are being underlined as if they are spelled incorrectly. With a right-click, Pages suggests they should be spelled in the British-English way ("decentralised").

The OS settings on my MacBook Air are set to UK English. Pages is set to US spellings (File > Advanced > Language and Region: English/United States).

Why is the spell checker in Pages flagging up these correct US English spellings?

How do I make sure Pages knows that correct US English spellings do not need to be flagged as incorrect?

All suggestions appreciated!